The research analyzed the macroscopic characteristics of settlements along the Tianjin Section. In the future, microscopic quantitative analysis of the in-site characteristics of the streets, lanes, and courtyards in these settlements can be made to explore the in-site language and spatial genes. In the future, the in-site development of these settlements can be researched according to the following aspects: (1) To conserve, inherit, and develop the settlements along the Tianjin Section, efforts should be made to give scope to in-site collaboration of the local management departments of all sections and strengthen the toplevel management mechanisms. In-site evaluation should be made of settlements to clarify the in-site attributes of the settlements, influenced by multiple in-site laws, and establish an inheritance system of in-site phenotypes. (2) On the basis of repairing the ecological corridor of China's Grand Canal, efforts should be made to make use of the functions of the settlements in the context of the cavernous body, integrating the green corridor, parks, and water systems of the canal into the settlements, and setting up an ecological security pattern for the settlements. (3) Extract the in-site genes from the perspectives of regional spaces, settlements, street and lane spaces, building spaces and environmental spaces, and form the technological mapping of an in-site creation of settlements along the Tianjin Section. (4) Strengthen thought of in-site development of settlements featuring "integration of culture and tourism", and designing high-quality routes of different themes and categories, implementing the development of characteristic towns and villages and invigorate the living, ecological, cultural and industrial spaces of towns and villages.

**Author Contributions:** Conceptualization, Y.Z., J.-W.Y. and G.-M.B.; methodology, Y.Z. and Y.L.; software, Y.Z.; validation, Y.L., G.-M.B. and Y.L.; formal analysis, Y.L. and G.-M.B.; investigation, Y.Z., Y.-Z.D. and G.-M.B.; resources, Y.Z. and Y.-Z.D.; data curation, G.-M.B.; writing—original draft preparation, Y.Z.; writing—review and editing, G.-M.B.; visualization, Y.Z. and G.-M.B.; supervision, J.-W.Y.; project administration, J.-W.Y.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** The paper is a phased achievement of the Tianjin Art Planning Project (E18007), National Undergraduate Training Programs for Innovation and Entrepreneurship<PHONE_NUMBER>04 and<PHONE_NUMBER>08) and Tianjin Ren'ai College Scientific Research Project (XX18004).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data derived from the current study can be provided to readers upon request.

**Acknowledgments:** This paper was financially supported by the Tianjin Art Planning Project (E18007), National Undergraduate Training Programs for Innovation and Entrepreneurship<PHONE_NUMBER>04 and<PHONE_NUMBER>08) and Tianjin Ren'ai College Scientific Research Project (XX18004). The authors are grateful to Hengyu Gu for the support on the spatial syntax technology he provided.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

**Siyu Wang 1, Qingtan Deng 1,\*, Shuai Jin 2 and Guangbin Wang 1**

**Abstract:** This study focuses on the assessment of historic city vitality to address increasingly fragmented urban patterns and to prevent the decline of livability in older urban areas. In 1961, Jane Jacobs theorized urban vitality and found the main conditions that were required for the promotion of life in cities: diversity of land use, small block sizes, diversity of buildings with varied characteristics and ages, density of people and buildings, accessibility for all people without depending on private transport, and distance to border elements. Jacobs' criteria for urban vitality has had an indisputable influence on urban researchers and planners especially in the Anglo-American context. This perspective has influenced the development of New Urbanism and similar planning policies, such as neo-traditional communities and transit oriented development, yet her theories have to be more substantiated in Asia's developing cities, especially in China's historic cities. In order to verify the significance of Jacobs' urban vitality theory in Chinese historic cities, we develop a composite measure of 16 variables of built environment, and we test it using GIS-sDNA in a historic city with an aging population and low-income in Qingdao. A systematic approach to urban spatial analysis allows us to provide a detailed spatial interpretation of a historic city form. The results emphasize that historic cities vitality, far from being homogeneous, followed a multi-centered distribution pattern, which is related to the previous European planning of the region, where a gridtype pattern was more likely to disperse urban vitality. The results can serve as a useful framework for studying the livability and vitality of different areas of the city in different geographical contexts.

**Keywords:** urban vitality; Jane Jacobs; built environment; GIS-sDNA; historic city

#### **1. Introduction**

The world is experiencing rapid urbanization, especially in developing countries, and while urbanization has significantly improved living standards, it has also brought about unprecedented urban problems, such as land Fiscal-driven urbanization, hollowing out of old urban areas, and "ghost cities" [1]. In this context, historic cities generally face double decay of social vitality and physical morphology, such as over-tourism, gentrification, aging and impoverishment of aborigines, aging infrastructure, and a deteriorating living environment [2–4]. Historic city vitality is an isomorphism of the urban morphology and the social activities behind it [5], determined by the spatial distribution pattern of the elements of the historic city form [6,7]. The unique urban scenes of the historic city have often undergone centuries of shaping and are significantly different from what is commonly thought of as vibrant urban areas [8,9]. With rapid urbanization, the original functional types and spatial patterns of the historic city can no longer meet the demands of modern life and require necessary adjustments to meet the needs of residents' lives and the needs of regional development. Numerous studies have shown that the distribution of urban vitality can be significantly influenced by the planning of urban form elements [10–14]. Therefore, in different urban contexts, how to evaluate the vitality of historical cities more rational and how to propose targeted strategies to enhance them has become a major research problem.

**Citation:** Wang, S.; Deng, Q.; Jin, S.; Wang, G. Re-Examining Urban Vitality through Jane Jacobs' Criteria Using GIS-sDNA: The Case of Qingdao, China. *Buildings* **2022**, *12*, 1586. https://doi.org/10.3390/ buildings12101586

Academic Editors: Michael J Ostwald and Ju Hyun Lee

Received: 15 August 2022 Accepted: 25 September 2022 Published: 1 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

"The Death and Life of Great American Cities" (Jacobs, 1961) provided a landmark for urban planning theory with abiding influences [15]. This perspective is considered the benchmark for understanding how neighborhood vitality works today [16]. The mix of building functions and densities that meet the needs of citizens is fundamental to the urban vitality [17]. Her criteria have been recurrently rediscovered and revisited to respond to different needs and from different contexts and are still relevant in the present day. The outbreak of the COVID-19 has once again led to a consideration of Jacobs' theory and the revival of urban vitality. Observing the changes of the epidemic, it can be seen that traditional unit communities within historical cities have better epidemic prevention effects due to their relatively open boundaries and the diversity of functions within the community, carrying compounded functions within the residential units, than, for example, new commercial housing communities on the outskirts of large cities. Cities and other public institutions around the world are also explicitly or implicitly using her principles as part of planning strategies for the revitalization of local historic cities [18]. As a result, her theories have also received increasing attention from urban researchers around the world, who use her general view of urban vitality as a framework for reflecting on the success or failure of urban planning in ensuring urban quality [19]. The current urban vitality study aims to validate her ideas by assessing the degree of mixing and spatial distribution of buildings and facilities in the city [20–24].

In terms of the analytical approach to urban vitality, Jacobs argues that the city is an ever-expanding network of mobility, with population expansion, size, and traffic congestion problems expanding outward as the city itself becomes saturated [15]. Urban space is a collection of information flows and networks, and a growing number of studies prove that many urban phenomena are essentially network phenomena, such as the hierarchical scale, resilience, agglomeration, or scale efficiency of cities [25,26]. In network analysis methods, spatial information plays a leading role, containing location data, allowing us to study the physical underpinnings that govern and shape the flow of human information in urban spaces. [27]. More recently, the centrality assessment model and space syntax analysis have been used to evaluate the structural properties of street networks in an urban system. Street centrality indices representing closeness, betweenness, and straightness capture the skeleton of the urban system; these factors shape economic activities and land use intensity [28,29]. A research team from Cardiff University, UK, developed the spatial design network analysis (sDNA) tool [30], which proposes a line segment model that is based on the spatial syntax axis model, while the topological relationships between streets can be weighted with information such as street length and geometric angular distance to reflect real geospatial information, while compensating for the failure of spatial syntax to capture physical isolation and network efficiency, especially the navigation difficulties and psychological barriers of pedestrians [31]. Jacobs' vitality criteria are directly mirrored in sDNA, which incorporates four important indexes (density, connectivity, closeness, and betweenness) that are hypothesized to affect urban vitality in an urban system. In China, driven by both industrialization and urbanization, numerous satellite towns, industrial zones, commercial centers, and residential areas have emerged at the historic city fringe and pose significant challenges to sustainable urban development [32]. Until recently, empirical evidence in developing countries has indicated that the dynamic process and determinants of urban vitality can be different from the case studies in the United States or Europe due to their dissimilar historic urban morphology and territorial spatial planning policies. A lot of historic cities launched large-scale street planning projects and replicated the modern style of broad and grid roads from the US, thereby reducing their urban vitality [33].

The following questions are posed in this study: (1) What are the implications of Jacobs' criteria for urban vitality in the context of historic cities? (2) How to construct the evaluation index system of "JANE Index" of Asian historical cities based on Jacobs' criteria? (3) How to visualize and analyze the vitality of Qingdao's historical city through GIS-sDNA and other related methods, and propose targeted vitality regeneration strategies? Therefore, this study aims to enrich the existing empirical studies by assessing the level of vitality that

is exhibited by each neighborhood of a historic city and formulate strategies to enhance urban vitality.

#### **2. Jacobs' Criteria of Urban Vitality**

Jacobs defines urban vitality as "the production of a diverse urban life consisting of human activities and living places" [15]. She proposed four main conditions for the creation of urban vitality: diversity of land use, small block sizes, diversity of buildings with varied characteristics and ages, and density of people and buildings, as well as two secondary conditions: accessibility without reliance on private transportation and distance from boundary elements [20].

Land use diversity, similar to architectural diversity of different characteristics and ages, is seen as paramount to maintaining urban vitality. She believes that the mix of functions creates spatial differentiation and increases the efficiency of the use of urban facilities, such as reducing commuter traffic and increasing the potential for public transportation use in a high-density environment [34,35]. On the other hand, building diversity also reflects the diverse mix of socio-economic groups [36]. Jacobs paid close attention to the "old building needs" of urban neighborhoods and argued that the healthiest areas of the city need not only old buildings, but also new buildings that are interspersed with old buildings. She noted that old buildings and new buildings require different levels of economic gain, i.e., the economic return that is generated in the building, and that new businesses tend to naturally emerge in buildings with low economic overhead. Th diversity index measures the degree of concentration, mix, and proportional relationships by measuring the number of different building types at a given scale [37]. The diversity index makes a morphological description, which calculates the number of buildings, number of functional types, and evenness in a site unit, quantifying the degree of spatial diversity [38]. When urban facilities show a high number and type with even distribution in space, this pattern is positively correlated with urban vitality.

Small-scale street blocks are based on the need for social spaces, which helps to create connections between people in specific areas of the city. This index is used to measure the connectivity characteristics of the street network, such as the number of intersections, the average distance between intersections, and other variables [39]. Studies have shown that potential traffic routes can be increased if the distance between intersections in a street network is shorter and the number of intersections is higher at the same scale. However, too short intersection distances with too many intersections can lead to a larger road network footprint and adversely affect the layout of other facilities [40]. A reasonable proportional and quantitative distribution of small-scale streets is positively correlated with urban vitality. Connectivity is only possible when streets vary in size and each size has a reasonable proportion of streets.

The influence of building and population density on vitality builds on the above indexes and emphasizes the degree of aggregation of population and activity places at a certain spatial scale [41,42]. The higher the building density, the higher the pedestrian traffic on the street. However, dense land occupation should not be confused with the construction of large apartment buildings, and areas with high housing density are often seen as a disadvantage. The presence of public services, commercial facilities, etc., is related to Jacobs' concept of the "eye on the street," which means that the constant presence of people creates a "natural surveillance" system that has also been shown to improve safety [43]. Compared to large public spaces, small infrastructures can be considered as potential social places. The POI kernel density calculation method has proven to be a valid method for vibrancy evaluation, for example for recreational facilities, small restaurant facilities, green infrastructures, and other types of calculations [7,44,45].

In terms of accessibility as one of the two supplementary conditions for urban vitality, in contrast to automobile-driven urban planning, studies have generally referred either to the supply of public transportation on the one hand, or to walkability conditions on the other [46]. In terms of walkability, sDNA's closeness index measures walkability within a

specified radius. Boundary vacuum refers to the segregated forms of urban existence, in the form of barrier surfaces or lines, including functionally homogeneous urban spaces (i.e., large parks), natural elements that could act as barriers (i.e., rivers), and ground-level heavy transportation infrastructures (i.e., railways). Jacobs argued that these elements adversely affect urban vitality through man-made impermeable boundaries [22,46].

These six criteria that are necessary for the generation of urban vitality, as proposed by Jacobs, form the basis of the theoretical approach that was analyzed in this paper. Jacobs' influence on urban theory is most evident in Anglo-Saxon contexts. Her work has also inspired European, South American, and Asian urban studies on how to respond to urban development issues, such as the case studies of Barcelona, Cyprus, Santiago, and Seoul [20,21,46,47]. Revisiting Jacobs' theory of urban vitality is necessary because her focus on the incremental revitalization of already urbanized areas, rather than the one-time redevelopment of master plans [36] is consistent with the focus of current considerations in developing countries to address the low vitality of historic cities that are brought about by rapid urbanization. Although most applied studies that are based on Jacobs' original evaluation indexes have been conducted for various purposes, the means of handling the evaluation indexes are basically the same, usually transforming them into multiple morphological variables, adaptively weighting the set of values, and overlaying them into a comprehensive urban vitality index.

#### **3. Methods**

#### *3.1. Study Area*

This study focuses on the historic city of Qingdao, China (Figure 1). Qingdao city is located in the southeastern of Shandong peninsula (latitude 35◦35 –37◦09 N, longitude 119◦30 –121◦00 E) in eastern China, with a total area of about 28 km2. The historic city area includes 14 historic conservation areas with a total area of 13.041 km2 and a core protection area of 6.394 km2, including 549 cultural relics protection units at all levels, 309 historical buildings, 1694 traditional style buildings. and 39 industrial heritage sites within the city area. The average population density is 8059 people/km2. The distribution of landscape elements in each historic conservation area varies greatly, which is related to the adjustment of urban form planning and industrial structure in different historical stages. Until the end of the 19th century, Qingdao was a rural area with agriculture and fishing as the mainstay. In 1898, German colonialists forcibly leased Jiaozhou Bay, and Qingdao then entered colonial city stage. Urban planning of Qingdao was dominated by German colonialists, and the layout of city conformed to the topography and climate of the seaside hills. The urban residential area was divided into European and Chinese areas. The European area was spread along the southern coastal flat area, and the nature of the land was mainly for European residence, leisure, and entertainment, with outstanding landscape conditions. The Chinese area was located in the northern area of the city, including Dabaodao district, Taidong town, and Taisi town. Based on the unequal colonial zoning pattern of Germanoccupied Qingdao, there are huge differences between the European and Chinese districts in terms of urban location, plot scale, road width, building density, green landscape, and other aspects of the living environment. In 1902, the population density of the European areas was only 19.1 people/ha, and building density was only 20–25%; the population density of the Chinese areas was 417 people/ha, and building density was over 75%. In terms of green space, the European area accounted for 72.6% of the city's greenery, while there was no public green space within the Dabaodao district. Urban planning and construction in Qingdao during the German occupation period (1897–1914) established the spatial morphological diversity of Qingdao's historical urban area, resulting in six typical morphological types (Figure 2): (a) south of Guanhai Mountain (former European area), (b) south of Zhongshan Road (former European area), (c) Guanhai Mountain (former European area), (d) Dabaodao (former Chinese merchant settlement, (e) Taidong Town (former Chinese laborers settlement), and (f) Taisi Town (former Chinese poor settlement).

**Figure 1.** Study area. (**a**) Qingdao city area. (**b**) Location of Qingdao Historic City in Jiaozhou Bay. (**c**) Scope of Qingdao historic city.

**Figure 2.** Urban morphology types in Qingdao historic city.

We choose Qingdao as a case study of the application of Jacobs' theory to Asian cities because Asia, as the origin of human civilization and the most populous continent in the world, has accumulated a profound urban culture during the course of history, and its urban morphological development has the following characteristics: 1. High-density concentration of population and buildings, accompanied by a long-standing phenomenon of large differences in scale, class, and form in the central area. 2. Since 1897, Qingdao, as a modern colonial city, has experienced colonial urban planning led by Germany and Japan, and its urban development has both a technologically advanced industrial and trade base and a unique urban regional culture, with distinctive power in terms of street texture, neighborhood scale, green space ratio, and it has distinctive power and hierarchical differences in terms of street texture, neighborhood scale, green space ratio, building density, etc. It conforms to the general characteristics of Asian historical urban form while maintaining a unique regional culture.

#### *3.2. Variables and Data Sources*

This paper is based on Jacobs' theory of urban vitality, focusing on built environment characteristics, i.e., street form, land use, and human activity factors, adapted to the urban context of Qingdao, China, and translates Jacobs' theory into the general empirical model in Equation (1). Other variables were combined based on recent literature exploring vitality and its drivers in the Jacobs framework. The result is the adjusted JANE index, which includes 16 indicators that were compiled from different data sources. The model is based on the JANE index of urban vitality that was first constructed by Xavier Delclòs-Alió (2018). Table 1 details the variables that were used to construct these six dimensions and their respective data sources. Most of the data were obtained from official sources, while the rest were collected from spatial analysis by the research team using a geographic data system.

$$I\text{ANE Index} = f(\text{Div}, \text{Sca}, \text{Div}\_{\text{A}}, \text{Con}, \text{Acc}, \text{Bou}) \tag{1}$$

*Div* = Function diversity, (land use mixture, residential and non-residential ratio, commercial and facility ratio)

*Sca* = Road scale, (block size, road length density, connectivity and betweenness)

*DivA* = Building age diversity, (average building age, building age diversity)

*Con* = Concentricity, (building and floor area density, public facility density, people density)

*Acc* = Accessibility, (road closeness, distance to public transportation)

*Bou* = Distance to large single-use buildings, large parks, surface large roadways, parking areas, and empty lots.

Building function diversity (*Div*.) is defined as the presence of land use mixture (*D*1), residential and non-residential ratio (*D*2) and commercial and facility ratio (*D*3). In this study, based on the calculation method of information entropy, the Shannon–Weaver diversity index was used to calculate the land use diversity of spatial units using the main 11 land use function POIs (residential, commercial, work-related, recreational, and others) that were extracted from Amap, where n represents richness and *pi* with proportional richness. When all building types in the dataset are the same, all *pi* values are 1/n. The larger the difference in building type richness, the larger the weighted geometric mean ln *pi*, and the smaller the corresponding *D*1 value. If practically all the richness values are concentrated in one building type and the other types are very rare (even if they are numerous), the *D*1 value is close to 0. When there is only one type in space unit, the *D*1 value is equal to 0 [48,49]. The *D*2 index was created with the expression as in the table, where Resi refers to residential uses and NonResi to non-residential uses. Both indices take values from 0 to 1.

**Table 1.** Conditions and indicators that were used in the analysis.

Source: compiled by the authors.

For road scale (Sca), the indexes included block size (S1), road density (S2), connectivity (S3), and betweenness (S4). The street block size was first calculated by GIS, and then the road density, connection value, and penetration were calculated using sDNA with a radius of 15 min walking distance (1500 m). Where density indicates the length (*Li*) of a road within specified radius. The connectivity value indicates the sum of the street and all other connected streets (*Ki*), and the node with a higher connectivity value is the center of the street network. Usually the streets with more intersections are more connected and the ends are less connected. Betweenness is a measure of centrality that is based on the shortest path, which indicates the likelihood and frequency of a person passing a node in a street network movement [31,40]. Since people prefer to follow straight paths and angular distances in daily life because they are easier to remember and faster on average, this paper emphasizes the importance of angular distance (the distance that is measured as angular change) and prefers to use the shortest angular path rather than Euclidean distance. Then, we use the angle-weighted TPBtA metric from the sDNA theoretical framework for the measurement with the expression as in the table where N is the set of streets, Ry is the set of streets within the proposed radius from street y, and OD denotes the ratio of beginning to end in the penetration, between the interval (0, 1). z and y denote the two endpoints of the penetration path, and x denotes the measurement point of the street penetration degree. TPBtA measures the "through" potential of a street, which is positively correlated with house prices and rents, traffic flow, population density, and commuting flows in the street network [50].

As for building age diversity (DivA), Jacobs highlighted that cities need to ensure that buildings with different characteristics are present in order to guarantee a certain degree of socioeconomic diversity. "Plain, ordinary, low-value old buildings" are critically needed environments for small, entrepreneurial businesses and healthy districts and cities" [36,51]. With this intention, we first included building age of construction (DA1) and diversity of building age of construction (DA2), which correspond to the original concept of Jacobs. In Qingdao historic city, strict restrictions on the preservation of historic conservation areas, as well as phenomena such as tourism and gentrification, can clearly affect building age diversity. Too many old buildings can reduce the diversity of old and new buildings, thus affecting the self-organization of historic urban renewal and discouraging the generation of urban vitality.

In order to study the concentration criteria (Con), we have considered building density (C1), floor area ratio (C2), population density (C3), and public service facilities density (C4). Building density is the ratio of building projection area (*Mi*) to building land area (*SN*), reflecting the open space rate per unit space and the density of building coverage. The higher building density reflects the higher degree of intensive use of urban space. The floor area ratio is a concentrated reflection of the development intensity of urban space, and a higher development intensity means a higher degree of land use. The population density was calculated from Landsat Enhanced Thematic Mapper (ETM) satellite images with a 100 m spatial resolution provided by Worldpop. Public service facilities density is calculated by kernel density tool of ArcGIS. The principle is to define a threshold range (a circle of radius r) that is centered on the location of selected elements, and the kernel density value reaches a maximum at each core element ni and decreases as it moves away from ni until it drops to 0 when the distance from ni reaches the threshold r. Due to the properties of buildings that are distributed along roads, this method is suitable for analyzing the concentration characteristics of infrastructure points in urban networks [44,52]. In this study, kernel density is considered as a cluster characterizing the concentration, and the peak area indicates the area of vitality hotspots, and the expression is shown in the table, where k is kernel function, r is distance decay threshold, n is the number of elements in threshold range, and (x-xi) is the distance from point x to sample xi. The size of the bandwidth has a significant impact on the accuracy of the analysis results [53]. In order to accurately reflect the concentration characteristics and to consider the discrete degree of building distribution and its average influence range, the bandwidth is set to 1 km in this paper.

Accessibility (Acc) is defined by the closeness of the road network (A1) and pedestrian accessibility of transportation facilities (A2). Closeness calculates the accessibility of the spatial structure of the street network through sDNA, which measures the difficulty of directing each street within a given radius to all possible destinations. A path with high closeness usually has high accessibility and it is easier to reach a further place, so that people can more easily reach the space from the surrounding areas for activities and social interactions, thus stimulating space vitality [40]. In this paper, the angle-weighted NQPDA model in the sDNA theoretical framework is used as the expression of closeness, as shown in the table, where dM (x, y) denotes the shortest angular distance from street x to y based on the metric system. Rx denotes the set of polylines starting from street x within the proposed radius. W(y) denotes the weight of polyline y. P(y) denotes the ratio of any polyline y within the radius, when in discrete space, P(y) = 1 if the point is within the search radius, otherwise P(y) = 0; in continuous space, it is determined according to the ratio of radius to the length of segment, 0 ≤ P(y) ≤ 1. NQPDA can reflect street accessibility and mobility potential and is closely related to diverse land uses [54]. Pedestrian accessibility of transportation facilities measures the convenience of pedestrian access to public transportation services and is mainly related to the distribution of public transportation facilities (e.g., bus stops, subway stations, etc.) on a spatial scale. If the public transportation stations are distributed within the scale that is comfortable and reachable by walking, the higher people's willingness to choose public transportation; otherwise, the willingness will decrease. In this paper, when discussing pedestrian accessibility, we mainly choose rail transit. Considering the quickness of rail transit station transfer, this paper takes the straight-line distance between each spatial unit and its nearest rail station as the A2 index and uses the ArcGIS nearest neighbor analysis tool for spatial analysis, and the expression is shown in the table, where Dip is the spatial linear distance between spatial unit i and rail transit station, and then the minimum value Min (Dip) is taken among all the linear distances. D0 is the minimum distance constant, due to the different spatial scales of the whole study area, the range of values is different, this paper takes the value of 1500. If the distance between the spatial unit and the rail station is smaller, the larger A2 is.

Lastly, we incorporate in the analysis distance from border vacuums (Bou), taking into account single-use buildings (5000 m2 and above), large transportation infrastructures, single-use extensive service or administrative buildings, and also large parks (5000 m2 and above).

#### *3.3. Data Processing and Calculations*

For the study area, we selected the conservation planning area (2020–2035) of Qingdao historic city given by Qingdao Natural Resources and Planning Bureau, and firstly corrected the projection coordinate system for the main road layers in the road network that were extracted by OSM, and then merged and physically interrupted them to generate a total of 5949 street links and checked all the nodes. In the second step, the spatial unit division of the traffic analysis zone (TAZ) is used to convert the road network into polygon elements and to delete, trim, and merge the irregular road network in order to partition the study area into irregular polygons (Figure 3a). In addition, the fragmented parcels with very small areas were merged with neighboring parcels for the convenience of statistics and calculation, and the study area was divided into 2281 study units after final processing. In the third step, this paper selects and reclassifies a total of 11 functional types including residential land, land for public administration and public service facilities, land for commercial service facilities, land for roads and transportation facilities, and land for green spaces and squares according to the Urban Land Classification and Planning and Construction Land Standard (GB50137-2011) that was issued by the Ministry of Housing and Urban-Rural Development of China, including 10,301 for shopping, 7451 for living services, 7384 for restaurants, 3100 for residential areas and services, 2633 for transportation facilities, 1891 for culture and education, 1860 for medical facilities, 1252 for government offices, 848 for financial services, 834 for recreational facilities, and 215 for natural landscapes, totaling 37,769 POI data for spatial analysis (Figure 3b).

The number of each type of POI in each study unit and within 10 m of the boundary was counted in diversity analysis, and the Shannon index was calculated for each unit. Subsequently, we standardized the data for extreme differences in view of the diversity of each variable unit, divided all the results of index quantification into 8 categories in ArcGIS using the natural discontinuity method [55], and finally summed up to obtain the graded results of diversity evaluation of each study unit (Figure 4).

In the calculation of the street scale and accessibility, the road data were vectorized and preprocessed through processes such as coordinate correction on the ArcGIS platform. By breaking all of the "link" intersections and checking all of the intersections, we obtained 5949 road network segments. To remove the errors in the network, we ran the prepare network tool on the transport vector layer in the sDNA toolbox and obtained a road

network spatial database of 4863 road network segments. Finally, we performed "integrated analysis" on the output model, choose "Angle" weighting as the calculation type in the parameter settings, and set the analysis radius to 1500 m to calculate the local spatial scale, where the radius type is set to "continuous space" to obtain more accurate calculation results. The output of the integrated analysis is the result of the spatial syntax analysis of each index (Figure 5).

**Figure 3.** Required dataset type. (**a**) Road network distribution. (**b**) POIs distribution.

**Figure 4.** Building function diversity analysis. (**a**) Land use mix. (**b**) Residential and non-residential ratio. (**c**) Commercial and facility ratio. (**d**) Building function diversity composite index.

**Figure 5.** Road scale analysis. (**a**) Block size. (**b**) Road centerline density. (**c**) Road centerline connectivity. (**d**) Road centerline betweenness. (**e**) Road scale composite index.

In the building age diversity analysis, we used urban real estate statistics and connected them to TAZ units to obtain Figure 6. For the concentration analysis, this paper uses 100 m resolution open space population density statistics that were provided by WorldPop opensource dataset. There were four public service facilities of transportation, culture, education, and healthcare that were selected, including 2659 POIs of transportation service facilities, 215 cultural service facilities, 1919 educational service facilities, and 1866 medical service facilities, totaling 6659 POIs of basic service facilities. Then, we imported the obtained data into ArcGIS and carried out the kernel density calculation of point elements separately. Among the parameter settings, the image element size was set to 5.6 m, and the search radius was set to 600 m based on the 10-min walking radius. Finally, we used the entropy weight method to assign weights to the above four POI types, and assigned 0.0956, 0.4427, 0.1488, and 0.3128 weight ratios in turn, and overlapped with TAZ units to analyze the grading results of the comprehensive evaluation of concentration (Figure 7).

**Figure 6.** Building age diversity analysis. (**a**) Building construction age. (**b**) Building construction age diversity. (**c**) Building age diversity composite index.

**Figure 7.** Connectivity analysis. (**a**) Buildings density. (**b**) Floor area density. (**c**) People density. (**d**) Public facility density. (**e**) Concentration composite index.

In order to combine the 16 constituent indexes with the corresponding urban vitality criteria, we weighted them by applying the raw z-values. Among the six dimension indicators, Jacobs regards the first four as the primary conditions for the generation of urban vitality. She believes that these four conditions must work together to generate urban vitality, and that the absence of any one of them will prevent the generation of vitality, so we use equal weights for the first 4 indicators. The last two conditions are considered as secondary conditions that are more independent of each other, but they also play an important role. In this paper, we adopt a more direct idea of weight assignment, i.e., 2:2:2:2:1:1, which is a simpler and more direct expression of Jacobs' criterion. The JANE index of urban vitality after overlay analysis is constructed as shown in (16).

$$I\text{-}I\text{-}N\text{-}Index = \frac{1}{5}(\text{Div}) + \frac{1}{5}(\text{Div}\_A) + \frac{1}{5}(\text{Sca}) + \frac{1}{5}(\text{Con}) + \frac{1}{10}(\text{Acc}) + \frac{1}{10}(\text{Bou})\tag{16}$$

#### **4. Results**

The result of mapping Jane Jacobs' urban vitality criteria in Qingdao historic city is presented in different maps in Figures 4–10. Comments on the side are made for each of the urban vitality conditions as well as for the final result in order to support their spatial interpretation.

#### *4.1. Building Function Diversity*

The spatial distribution of function diversity corresponding scores in Qingdao historic city is presented in Figure 4. Figure 4a–c show the spatial distribution of land use mixture (D1), residential and non-residential ratio (D2), and commercial and facility ratio (D3), respectively. The high values of D1 are mainly located in the central–eastern part of the country, with relatively low diversity in coastal land use, while D2 shows the opposite distribution pattern, with a relatively high ratio of residential to employment in the coastal zone. Figure 4d shows the weighted superposition of the three indicators using the entropy weighting method to obtain the composite index of Div.

Diversity was tested for spatial autocorrelation, given that the Global Moran' s I was 0.29 and the z-score was 25.72, thus implying significant spatial correlation and clustering. The pattern of building function diversity in Qingdao is more complex: (1) The spatial

pattern of the high-value area (Div ≥ 0.70) follows a banding pattern along Qingdao Jiaozhou road, and the further away from the main road, the more obvious the diversity decay, with a mean diversity index (Mdiv) of 0.77, including mean land use mix (MD1) 0.75, mean residential-nonresidential ratio (MD2) 0.31, and mean commercial-public facility ratio (MD3) 0.82. Due to the limitation of the conservation plan of Qingdao's historic city, the diversity of Zhongshan Road and Taidong streets are low and unevenly distributed, compared to the higher diversity of land use in the surrounding urban areas. (2) Anselin Local Moran' s I test shows that some areas show significant differences in land use mix within a relatively short distance, such as the intersection of Laiyang Road and Qingyu Road and Huiquan Commercial Square, where there are many high-low outlier areas, which are characterized by high values of local diversity in single-use building functional areas. This contrast is further revealed by the disaggregated nature of the raw data, and this micro-contrast is likely to be weakened if larger spatial scales (e.g., municipal scales) are used. (3) The area with low diversity value (Div ≤ 0.30) is mainly located in the eastern mountainous region with a mean diversity index (Mdiv) 0.09, including mean land use mix (MD1) 0.15, mean residential-nonresidential ratio (MD2) 0.02, and mean commercial-public facilities ratio (MD3) 0.05, for instance, Taiping Mountain, Baguan Mountain, and Qingdao Mountain generally have lower land use diversity. Within the core area of the old city, the southern coastal neighborhoods such as Sifang Road, Guanhai Mountain, and Baguan Mountain reflect a certain degree of homogeneity in land use and a low diversity index.

In urban conservation planning of Qingdao that was formulated in 2020, Qingdao puts forward three categories of control requirements for construction height, the first of which requires the existing height to be maintained in the core protection area and no change is allowed. The second category requires that the construction height in the historic district of the main city shall not exceed 18 m, and in the towns of Taixi and Taidong shall not exceed 24 m, and the new buildings shall not exceed the adjacent historic buildings. The third category requires that the building heights in other areas shall not affect the overall mountain overlook view area and protect the overall view corridor. Combined with the results of the diversity distribution, the construction height restriction has a significant impact on the diversity of building functions, and the diversity of the main urban area is generally lower due to the 18-m building height limit, however, there are also individual phenomena, such as Parkson Shopping Center, Guangde Li 1898, and Yue Hik Lai Shopping Plaza in the Zhongshan Road neighborhood along the southern coast, which are much higher than the control height due to their early construction, and these buildings are affected by the conservation planning and development These buildings are affected by the conservation planning, development is reduced, and the customer flow is also reduced accordingly. The height limit in Taidong Town is higher than that of the main urban area, and thus has higher diversity characteristics, and it gathers more people traffic, however, because the central area of Taidong Street is stricter than the peripheral height limit, so the diversity distribution shows the characteristics of low center and high surrounding.

#### *4.2. Road Scale*

The spatial distribution of the road scale score is presented in Figure 5. Figure 5a to Figure 5d show the spatial distribution of block size (S1), road centerline density (S2), road centerline connectivity (S3), and road centerline betweenness (S4), respectively. The S1 high value area is mainly located in the port area along Jiaozhou Bay and the mountainous area in the southeast, and the street blocks are large in scale; the S1 low value area has a clear correlation with the S2 high value area, and the small scale street blocks increase the street density and have high centrality; S3 and S4 show the connectivity and passability of the local network. Figure 4d shows the Sca composite index that was obtained by weighted superposition of the four indicators using the entropy weighting method.

Road scale was tested for spatial autocorrelation, given that the Global Moran' s I was 0.85 and the z-score was 71.70, thus implying significant spatial correlation and clustering. Since walking is more suitable for stimulating urban vitality than vehicular traffic, we focus

on the spatial scale of people in the walking mode and set the search radius in the spatial syntax calculation to 1500 m, which is close to the 10 min walking circle range, covering central, daily, and active streets. According to Jacobs, smaller block sizes and shorter streets provide more favorable conditions for contact between people. The results of this index show two more significant spatial patterns: (1) High-value areas (Sca ≥ 0.52) identify areas that provide more public contact, which were planned during the German occupation period (1897–1914) in accordance with the European "garden city" concept of colonial planning, with a high-density road network, mainly in the central area of the old urban area, historic area of Taixi and Taidong, with a cluster layout. They are mainly located in the central area of the old city and in the historic areas of Taisi and Taitung and are laid out in clusters. These areas have a mean street-scale index (MSca) of 0.62, with a mean block area index (MS1) of 0.22, a mean road density index (MS2) of 0.73, a mean connectivity value (MS3) of 0.58, and a mean betweenness value (MS4) of 0.20. The newer urban development areas are considered to be areas with lower contact potential due to larger, more evenly spaced blocks planned and wider streets. (2) Low-value areas (Sca ≤ 0.30) that are located in the northern part of old urban center, which is bordered by Taidong historic area, and in southeast along Huiquan Bay and Taiping Bay, with a banding layout and mean street-scale index (MS1) of 0.20, including a mean block area index (MS2) of 0.67, a mean road density index (MS3) of 0.25, a mean connection value of 0.38, and a mean penetration (MS4) of 0.06. Various historic cities in the northern part of the city were planned during the massive land expansion program of the German occupation (1910–1914), with a low-density road network and lower contact opportunity values in these expansion areas compared to more compact road structures in old urban centers.

#### *4.3. Building Age Diversity*

The next criteria for urban vitality, building age diversity, allows us to distinguish the areas in which a larger presence of older buildings coexist with newer ones, from those that are newer and more uniform in terms of age. Building age diversity was tested for spatial autocorrelation, given that the Global Moran' s I was 0.63 and the z-score was 52.97, thus implying significant spatial correlation and clustering. The results for this index reveal higher scores within the city's historic center, where a larger number of old buildings are intermingled with more recent structures that were built as part of the densification that has been occurring over the course of the past 123 years. The results show that: (1) High-value areas (DivA ≥ 0.45) that are distributed in addition to the historical centers, form important peripheral sub-centers in the southeast coastal area with a mean building age diversity index (MDivA) of 0.49, including a mean building age index (MDA1) of 0.81, and a mean building age diversity (MDA2) of 0.55, such as Yu Hill, Signal Hill historic conservation area, Huiquan Commercial Square, Badaguan, Taiping cape, due to its favorable climate and its historical use as a European retreat area, have more intact architectural types, mainly single-family houses, which are now mostly utilized as tourist resources such as resorts and sea baths. (2) Low-value areas (DivA ≤ 0.18), which are mainly located in Badaxia Street west of the railway station, and Dengzhou Road Street north of East—West Rapid Road, etc., show greater homogeneity in terms of building age, a feature that constitutes a negative factor in urban vitality. The mean building age diversity index (MDivA) 0.04, where the mean building age index (MDA1) 0.07 and the mean building age diversity (MDA2) 0.22 are identified in the Taixi and Taidong historic areas and Liaoning Road. These areas have lower housing and rental prices than higher value areas due to the government's Affordable Housing, Housing Provident Fund (HPF) and Low Rent Housing programs since 1994, which subsidize land allocation and reduce taxes on the one hand, and provide financial assistance to individuals on the other, allowing lower income families to enjoy lower rents.

#### *4.4. Concentricity*

Figure 7a–d show the spatial distribution of building density (C1), The floor area density (C2), people density (C3), and public facility density (C4), respectively, which all show more obvious spatial differences. There is an obvious spatial mismatch between building density and population density, and Taitung Town, which has a high concentration of population, but fails to provide housing density that meets the demand. Public facilities show a multi-core distribution, and the same problem of uneven distribution exists. Figure 7e shows the composite index after the weighted overlay of the four indicators.

Concentricity was tested for spatial autocorrelation, given that the Global Moran' s I was 0.65 and the z-score was 55.23, thus implying significant spatial correlation and clustering. Similar to the road scale evaluations, smaller street-scales, greater road density, connectivity, and betweenness also explained to some extent the distribution of concentration indexes. (1) High-value areas (Con ≥ 0.31) correspond first to high population and housing density neighborhoods in the central area of the old urban area and north of it, with a mean concentration index (MCon) of 0.28, including a mean building density index (MC1) of 0.41, a mean floor area ratio index (MC2) of 0.13, a population density index (MC3) of 0.11, and a public facilities density index (MC4) of 0.53. The local Moran's index test (Anselin Local Moran' s I test) shows that this area is mostly a high-high cluster area. (2) Although low-value areas (Con ≤ 0.14) show no clear trend in their spatial distribution, these areas are generally located in the urban fringe zone, corresponding to industrial sites or areas that are constrained by construction due to topography or the presence of large nature conservation parks, with a mean concentration index (MCon) of 0.05, where the mean building density index (MC1) is 0.09, the mean floor area ratio index (MC2) is 0.02, the mean population density index (MC3) is 0.03, and the mean public facilities density index (MC4) is 0.16. However, the presence of localized high-value areas can also be seen in the surrounding low-value areas. One explanation for this spatial pattern is that these areas are laid out with a high density of specific public service facilities, such as the historic district of Guanxiang Hill and its surrounding neighborhoods, which are dominated by medical service facilities.

In urban conservation planning of Qingdao, the requirements for building volume, layout, color, material, etc., are consistent with the historical features, that is, the environmental characteristics of small volumes of white walls, red tiles, and green trees, which attract Although the renovation of historical buildings for small entertainment facilities such as coffee shops and bars has limited the attraction of mixed functions to a certain extent, it has improved the attraction of tourists, and thus improved the characteristics of concentration. Such an agglomeration is most obvious in Zhongshan Road, Yushan, and Signal Hill neighborhoods.

#### *4.5. Accessibility and Boundary Vacuum*

The spatial distribution of the two complementary conditions for urban vitality are presented in Figure 8. Accessibility is mainly explained by the "center-periphery" structural logic of spatial syntax, i.e., the geometric law of decay from the center to the periphery, and its spatial distribution was tested for spatial autocorrelation, given that the Global Moran' s I was 0.92 and the z-score was 77.71, thus implying significant spatial correlation and clustering. Accessibility of Qingdao historic city shows a multi-polar layout, basically reflecting the public transportation network, with the shorter average distance from the geometric center of the city indicating more convenient transportation. The results show that (1) high-value areas (Acc ≥ 0.70) are concentrated in central areas of the old urban area with Zhongshan Road as the central axis, areas around the railway station and Taidong historic area, with a mean accessibility (MAcc) of 0.80, including a mean closeness (MA1) of 0.69 and a mean traffic facility proximity (MA2) of 0.05. The central area is characterized by a lower slope compared to the mountainous area, and its road network is planned to follow the topography and there are traffic calming areas, so the accessibility becomes higher. Another reason is the location of the railroad station at the end of the Jiaoji Railway, which connects the urban space. Under the influence of the east-west fast road network, closeness and public transportation accessibility appear to be multi-polar, and the Taitung urban area becomes the trend of urban structure in the 21st century. (2) The low-value area

(Acc ≤ 0.30) is concentrated in port area along Jiaozhou Bay, mountainous area such as Taiping Mountain, and the southeast coastal area, with a mean accessibility (MAcc) of 0.19, including a mean proximity (MA1) of 0.16 and a mean transportation facilities proximity (MA2) of 0.12. This area locates at the edge of southeast urban area, where the road network is less segmented and more interrupted; reducing the distribution of interrupted street network will help create more "reach" opportunities for people.

**Figure 8.** Accessibility and boundary vacuum analysis. (**a**) Road centerline closeness. (**b**) Distance to public transportation. (**c**) Accessibility composite index. (**d**) Distance to boundary vacuum.

In terms of distance from border vacuums, areas with high values are those that are distant to large infrastructures or large, single-used buildings that can potentially discourage street life. These boundary vacuum elements are mainly the Haibo River, Changle River, and Hangan Viaduct at the northern city boundary; Jiao Ning Viaduct extending eastward from the central old urban center; and the railway lines around the railway station. In the surrounding zone, the inner part of the old urban area is also covered with major parking lots in points and large parks such as nature reserves in polygon form. The Bou index measures the distance of each spatial unit from boundary vacuum elements, and its distribution pattern is tested for spatial autocorrelation, given that the Global Moran's I 0.33 and the z-score is 27.81, so there are significant spatial correlations and clustering. The results show that: (1) High-value areas (Bou ≥ 0.63) are mainly located along the southeast coastline, including Signal Hill, Yu Hill, and Baguan Hill, due to the superior natural environment and distance from facilities such as railway lines and large parking lots. Besides, there are points of high value areas that are distributed along Liaoning Road and Changle Road in the north. (2) Although the boundary vacuum elements are mainly located at urban boundaries, it is worth noticing that within the central area of Qingdao's historic city, there are also numerous streets that are close to the boundary vacuum and present low values. Since the planning of the German system at the end of the 19th century was motivated by the construction of Qingdao as a military base and industrial port, large infrastructures such as railroad stations, Jiaoji railroad, and ports were planned, and these infrastructural elements had a negative impact on human contact.

#### *4.6. JANE Index in Qingdao Historic City*

We then synthesized the six conditions of urban vitality in the JANE Index. Higher values of the JANE Index correspond to areas with a higher potential for urban vitality, while lower values indicate the lack of such conditions. The distribution of Qingdao historic city JANE index is shown in Figure 9.

The potential of urban vitality in Qingdao historic city presents a polycentric pattern, as we find high values of the JANE Index distributed in different sub-centers of potential vitality. In turn, different intensities of urban vitality potential are identified. As Jacobs' advanced, not to be picked up at the city scale, but is at the district and neighborhood levels that this is properly understood. In this sense, one center and four sub-centers of urban vitality were identified in Qingdao historic city. (1) The west end of Qingdao East–West Rapid Road, the south end of Jiaoji Railway, Taiping Road, and Jiangsu Road enclose the central area with the highest urban vitality, and the north end has part of the junction between the south and north districts of Qingdao, such as Zhongshan Road Street and Sifang Street, with a mean JANE index of 0.49 and a standard deviation 0.11 and more uniform spatial distribution. The area has a long planning history, significant street morphological differences, and more historic buildings, and thus has more intensive crowd activity (Con), more neighborhood contact opportunities (Sca), higher building age diversity (Div\_A), and street accessibility (Acc). The general pattern of spatial patterns of urban vitality is again validated by the center–periphery logic: closer to the center means that higher urban vitality is achieved, while those areas with lower or no potential vitality are incorporated into peripheral contours of city. (2) The streets of Taidong, Liaoning Road, Badaxia and Jiangsu Road constitute the four sub-centers of urban vitality, with mean JANE scores of 0.44, 0.45, 0.43, and 0.39 and standard deviations of 0.08, 0.07, 0.07, and 0.08, respectively, and a more uniform spatial distribution. Compared with the traditional 100 m×100 m grid in the historic center, these areas have a narrower street pattern, smaller block scale, and high housing and population density, and higher JANE indexes are mainly explained by two indicators of concentration (Con) and street-scale (Sca). (3) Similarly, it is evident that some edge areas have high JANE scores, such as Badaguan Street near Wushengguan Road and Dagang Street near Dagang Weisi Road, which are mainly explained by the street-scale (Sca) index. These areas with high-low cluster values confirm the multicenter trend of urban vitality. 4) The area with a low JANE index (JANE ≤ 0.30) is mainly located in northern Dagang Street, Yunnan Road Street, and the southern end of Badaxia Street, with a mean JANE index 0.24, 0.29, and 0.31 and a standard deviation 0.03, 0.05, and 0.07, respectively, and more uniform spatial distribution. The lower spatial vitality of this area is related to its location in the urban edge area, relative isolation, large street block area, low density, and single land use.

**Figure 9.** JANE Index of urban vitality.

Figure 10 shows the grading results of the Qingdao historic city JANE index, and Table 2 shows the statistics of TAZ units at each level. There were four urban vitality classes of Qingdao's historic city that were identified by JANE cluster analysis, labeled as high vitality areas, moderate vitality areas, low vitality areas, and non-vitality areas. The analysis results show that high vitality areas account for about 6.73% of the total area of the study area, with a mean JANE index of 0.568, standard deviation of 0.048, and extreme values 0.507 and 0.699, mostly located in historic central areas, and a few areas in the above four JANE index sub-centers. These areas are characterized by a small-scale street network, a mixed distribution of old and new buildings, and a mix of commercial and public facilities, producing a high accessibility and diversity value, a feature that is further enhanced by the increased distance from the boundary vacuum. The main axis of urban vitality in this area is from Zhongshan Road southward to the Trestle Bridge, where traditional Qingdao neighborhoods, shopping malls, and Trestle Park converge, reflecting a high level of diversity and neighborhood access. On the eastern side of the old city, Guanhai Hill and the area to its south are highly accessible due to the radial road network, creating high vitality points. Finally, in addition to the historic center, the neighborhoods of Weihai Road Pedestrian Street in Taidong Street are characterized by a higher concentration of urban vitality due to higher population density and commercial facility density.

**Figure 10.** JANE index classification analysis.

**Table 2.** JANE index group statistics results.

Source: compiled by the authors.

Secondary areas of moderate vitality zones account for approximately 26.98% of the study area, with a mean JANE index of 0.449, a standard deviation of 0.032, and extreme values of 0.392 and 0.505. In terms of the planned street patterns, the area is expected to exhibit a high potential level of street life, but the combination with other indexes reflects that central areas of the district are either categorized as moderate or even low vitality zones and tends to be a transitional buffer zone between high and low vitality zones. Compared with the neighboring high vitality zones, the approximate street scale and population density show that the mixed degree of building functions and density become the main reason for the formation of transition zones. For example, among Taidong streets, the layout of Weihai Road pedestrian street tends to have a positive impact on local vitality, while neighborhoods that are farther away from pedestrian streets have relatively lower vitality.

Finally, areas that are classified as low and no vitality account for 29.21% and 37.08% of Qingdao's historic city, respectively, with low vitality areas having a mean JANE index of 0.338, a standard deviation of 0.028, and extreme values of 0.293 and 0.391, and no vitality areas having a mean JANE index of 0.248, a standard deviation of 0.033, and extreme values of 0.110 and 0.292, which are often are located in urban edge zones with low population and building densities and in close proximity to border vacuums. These two vibrant environments correspond to being located on the periphery of a moderately vibrant zone, presenting very low accessibility values. The analysis reveals that places that are in contact with agricultural, natural, or industrial areas tend to present significantly low values of urban vitality; on the one hand they usually are near extensive mountainous areas with significant slopes, such as Qingdao Mountain, Taiping Mountain, and Zhushui Mountain, and on the other hand they are located in functionally single areas with larger block scales, such as Dagang and Xiaogang in the north, and Taiping Cape and Huizhuan Cape in the southeast coast.

#### **5. Discussion and Conclusions**

This study aims to recontextualize Jacobs' urban vitality principle through urban spatial analysis and analyzes the livability and urban vitality of Chinese historic cities in the context of rapid urbanization. The newly established JANE index consists of six criteria with a total of 16 indexes, and since urban vitality is considered a key measure of urban residents' happiness and quality of life, this study contributes to a discussion of the nature of the drivers of urban vitality and the way that they are distributed in space. To this end, the study creates a framework of indexes based on the Delclòs-Alió establishment and integrates the theoretical and methodological proposals that were made by recent literature on the subject, and performs a spatial analysis based on GIS-sDNA, the results of which are applicable to Qingdao historic city with its different muscular morphology. Jacobs has a rather important role in the history of the field of modern urbanism. From a practical point of view, this paper will help planners to combine morphological conditions that are conducive to vibrant streets, urban environments, and neighborhoods, thus promoting community, local spatial practices, and neighborhoods.

The results of this study show that conditions of urban vitality in today's cities are not necessarily related to centrality, a specific urban texture, or a certain income level, but may be a result of different combinations of certain urban characteristics. In this sense, this study validates the applicability of Jacobs' theory to cities in developing Asia, where more street activity and vitality is not singularly concentrated in central areas, although these areas are often characterized by higher population densities and mixed uses. Qingdao's historic urban vitality follows a multi-centered spatial pattern, with a more fragmented distribution of public services and coexists with a dense network of local businesses. Different combinations of indexes of urban vitality are distributed throughout the area, showing a multi-centered pattern covering a diverse range of urban texture with two characteristics: (1) The main areas with high potential values of urban vitality in Qingdao's historic city correspond to the oldest historic city center, which has the original dynamics of all vitality indexes. The square grid street pattern that was planned along the topography

creates the conditions for urban vitality. As Jacobs argues, the mix of land uses and the distribution of more intersections produce a more compact and complex form that provides constant access to citizens and a diversity of facilities and services. (2) Medium and high potential vitality values are found equally in parts of the city that are away from the Old City Center, such as the Taidong and Taixi historic areas. It has long been marginalized due to topographical factors and the racial segregation motive in historical planning. In recent years, the area has gradually formed a sub-center of urban vitality in Qingdao with the combination of different vitality conditions such as increasing population density and diverse land use development, which attracts and directs not only short-distance mobility but also people that are traveling long distances.

After China established the historic and cultural city protection system in 1982, protection concepts gradually developed from single buildings to overall historic environmental protection in historic districts. Qingdao historic city was incorporated into the national historic and cultural city protection list in 1994, as Figure 8 delineates 14 historic conservation areas and 2 historic areas. Qingdao government strictly protects the physical environment of the historic city but implements relatively few policies and practices to improve the social life of residents and promote the renewal and maintenance of old public service facilities. Qingdao's historic conservation areas originated from the segregated division of European and Chinese districts in the urban planning during the German rule. Historic conservation areas, mainly in the former European district, are mainly villa residences, with large street block scale and high green space rate, but there is a problem of insufficient vitality, so in the renovation and reuse of such streets, commercial and public service facilities should be increased to improve the diversity of architectural functions. The former Chinese district is the main historical district that was originally for residential-commercial mixed function, with smaller street block scale, low green space rate, and poor livability. In the renovation and reuse of these kinds of streets, green space and public activity space should be increased. Based on the relationship between the Jacobs index system and the quantitative results, we established the following planning strategies to enhance the vitality of Qingdao's historic city:

There are certain limitations in this study. Firstly, urban vitality is a broad and complex concept involving social, economic, cultural, and other multi-dimensional contents. Although this study characterizes urban vitality by various parameters of LBS data with objectivity, it cannot accurately express the subjective perception of the public. For example, the satisfaction and perception of an active population on the space of historical neighborhoods mean the research results may be biased due to the data characteristics. Follow-up studies should calculate more accurate variable data that are based on streetscape images and explore human perception characteristics in detail. Second, POI data cannot present time information, a limitation that prevents the unique seasonal characteristics of historic urban vibrancy from being effectively measured. Follow-up studies can combine multisource spatio-temporal data such as social media punch card data and cell phone signaling data to establish a more accurate urban vitality index system.

**Author Contributions:** Conceptualization, S.W. and Q.D.; methodology, S.W. and Q.D.; software, S.W. and S.J.; validation, Q.D. and G.W.; formal analysis, S.W. and Q.D.; investigation, Q.D. and G.W., resources, Q.D. and S.J.; data curation, S.W. and S.J.; writing—original draft preparation, S.W. and Q.D.; writing—review and editing, Q.D., S.W. and G.W., visualization, S.W., S.J. and G.W.; supervision, Q.D.; project administration, S.W. and Q.D.; funding acquisition, Q.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Data derived from the current study can be provided to readers upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Reshaping Publicness: Research on Correlation between Public Participation and Spatial Form in Urban Space Based on Space Syntax—A Case Study on Nanjing Xinjiekou**

**Mengyao Pan 1, Yangfan Shen 2, Qiaochu Jiang 1, Qi Zhou 1,\* and Yinghan Li 3,\***

**Abstract:** This paper focuses on urban regeneration practices in central urban areas, aiming to find key points for reshaping the publicness of urban spaces by exploring the morphological features of public spaces and the spatial distribution patterns of public activities. Now that China's urbanization process has stabilized, large-scale regeneration is no longer applicable to the current urban environment, and urban morphology has proved to be significantly useful in understanding and designing the built environment. However, current research lacks quantitative studies on morphology and public activities, and thus is hardly instructive for the cognition and design of spatial morphology in specific locations. Therefore, this paper attempts to subdivide spatial morphology at the level of "microrenewal" or "micro-renovation" in order to explore the impact of spatial morphology on public participation in cities. The site chosen for this study is Xinjiekou in Nanjing. As a key area of two important arteries in the center of Nanjing, Xinjiekou has been a gathering place for a variety of commercial forms, such as finance, retail department stores, restaurants, and entertainment, and has been the commercial and financial center of Nanjing since the 1940s. In an on-site observation of urban development and pedestrian flow in the Xinjiekou area, the study found that despite its status as the area with the highest degree of spatial accessibility and public participation, its public space has gradually lost its attractiveness to residents, who lack a sense of participation and place identity. Based on the study of urban public spaces, both accessibility and choice play an important role in increasing public participation. Therefore, this study combined observation and quantitative analysis of Space Syntax to obtain the distribution of accessibility, choice, and public activity. Based on the results of the analysis, this research uses GWR as the statistical method to clarify the correlation between different variables. The final conclusion is that when the space type is a path with high choice value and the paths are connected, the enhancement of accessibility and choice plays an important role in promoting public participation. This statistically based empirical study of testable correlations is very helpful for the perception of location-specific spaces with high levels of interpretability and confidence. Thus, it further guides the design and has a high reference value for future spatial planning.

**Keywords:** urban renewal; accessibility; place-making; typology; Space Syntax

#### **1. Introduction**

In recent decades, the public nature of public spaces has been diminishing globally, suggesting that public space is losing its potential to provide opportunities for social interaction. Some superficial explanations suggest that the possible reasons for this stem from the suppression of the creative sphere in the context of homogenization constructed by globalization. Individuality and uniqueness have been destroyed and urban spaces tend to be similar. As described in the Beijing Charter, "a thousand cities all look alike" in the eyes of people today [1]. This is the dilemma facing today's cities, as the public memory carried

**Citation:** Pan, M.; Shen, Y.; Jiang, Q.; Zhou, Q.; Li, Y. Reshaping Publicness: Research on Correlation between Public Participation and Spatial Form in Urban Space Based on Space Syntax—A Case Study on Nanjing Xinjiekou. *Buildings* **2022**, *12*, 1492. https://doi.org/10.3390/ buildings12091492

Academic Editors: Michael J. Ostwald and Ju Hyun Lee

Received: 2 August 2022 Accepted: 16 September 2022 Published: 19 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

by distinctive buildings and neighborhoods in urban public spaces is being destroyed with the rapid development. This in turn further promotes the loss of the meaning of public space, and gradually makes the public space in modern cities a meaningless transitional space where people find it difficult to develop a sense of belonging.

Many scholars have studied the publicness of urban public space in terms of its ownership, cognition, management, and public activities. A typical one is the Project of Public Space (2010). Based on a study of thousands of public spaces, this project concluded that a highly public space should have the characteristics of Social activities, Comfort and image, Accessibility, and Sociability to facilitate the interaction between the residents and the space. This view is not alone, as Arentze, Afonso, Ye et al. argue that the daily activities of individuals/families are driven by a set of universal needs, and thus travel time, comfort, and sociability are important factors that drive public engagement in public spaces [2–4]. At the same time this process of social–spatial interaction as a dynamic flow plays a considerable degree of permeability to its neighboring spaces, and thus on this level good public spaces will prove influential to neighborhoods beyond the limits of its own physical space. This idea is also similar to the one that was proposed by Jane Jacobs and William H. White in the 1960s in response to spatial deactivation—namely, the use of place-making, i.e., the use of a community's assets, values, and resources to create a place that promotes people's health, well-being, and happiness in order to enhance their sense of identity and the meaning of place [5–10].

The most prominent number of these related studies for spatial accessibility and morphology, Ye and Le Texier conclude that accessibility plays a crucial activity in public engagement, and the morphology of urban spaces will largely influence people's choices [4,11]. Although many studies have investigated public spaces in detail, these studies have not explored the association between public participation (publicness) and accessibility, choice and urban form, using a quantitative approach for specific sites from a design perspective. In addition, there are fewer studies on the quantitative analysis of spatial morphology, and the challenge of constructing correlations with actual social phenomena leads to difficulties in applying the results of the analysis to design.

At the same time, in the context of China's urban public space, the density, intensity, and height of the built environment have increased dramatically in the urbanization process since the implementation of the reform and opening-up policy 40 years ago [12]. The massive development and construction of urban centers has compensated for the lack of commercial space in the original areas. However, excessive commercialization has encroached on public space, while people are attracted to commercial services and neglect public space during their walks [13,14]. Commercial spaces pre-screen residents due to their ownership and the consumption or services they provide. As a result, there is a discontinuity and disintegration of spatial experience for some residents. This reduces the interaction between the individual and the environment, thus re-diminishing the resident's identification with place [15,16].

As Rowan Moore argues in his book *The Slow Born City*, cities should change gradually, with a combination of private investment, public interest, and legislative action, rather than destructive change. Therefore, this study will interpret the evolution of urban spatial forms in the process of urban development in the context of "micro-renewal" with a Space Syntax in order to obtain the basic characteristics of urban forms and spatial accessibility and choices. Secondly, we make long-term observations of the site and map the location of public activities. By calculating the correlation between these two types of spatial elements, the results are finally projected onto the different morphological types of urban public space to determine the role of urban morphology in promoting public participation. Based on this, this study attempts a combined qualitative–quantitative approach to public space, exploring the significance of spatial design in leading to positive human well-being, and the demonstration and testing of the feasibility of design potential. The lack of empirical analysis of spatial planning is remedied.

#### **2. Literature Review**

This chapter focuses on the definition of urban public space and the social significance it carries. On the basis of this chapter, the research objectives of this paper are combined with the research perspectives of choice, accessibility, and typology for the study of public space. Finally, it summarizes the Space Syntax and related research methods.

#### *2.1. Publicness of Urban Space*

Place and space have long been the focus of discussion in academic circles. In the context of urban design, the two can be used alternatively. Place is considered to be a space bearing social relations and identity [6–9]. Therefore, public engagement is particularly important in urban design. Lefebvre believes that active space (place with functions and meanings) is the result of people using space to implement daily necessities [17]. Urban design in turn endows place with meaning to reshape the daily life of the public.

As one of the first scholars to introduce the term "public space" into the field of urban study, Jacobs strongly criticized the functionalism-based urban reconstruction policy of American cities in the 1950s in *Life and Death of American Big Cities*. She believes that modernist planning has destroyed the vitality of urban streets and urban fabric in traditional cities, turning closely related urban blocks into isolated architectural individuals. The function-based division of urban space destroys the social value of space. The planner and designer must advocate the development of "public space" in "urban space", and take public space as a key element to promote the formation of good social exchanges and restore urban vitality in urban development and community construction [8]. In her view, the value of public space lies in that its existence can promote the communication of people from different social strata or groups in the city. Because of its nature of diversity and inclusiveness, public space plays a vital role in promoting social mutual understanding and integration as well as social stability and harmony, and serves as an important source of urban vitality.

The past 40 years have witnessed a greater engagement of factors such as capital, power, politics, and economy in the development and reform of contemporary Chinese cities. The philosophy of "place-making" echoes humans' basic needs for a high-quality living environment. According to Schultz, creating a place means expressing a sense of existence. This concept reflects the phenomenological essence of place-making. People realize their "settlement" in the world by creating place [18]. Therefore, providing people with a recognizable place that gives them a sense of belonging is the fundamental purpose of place-making. This purpose can be achieved, in part, through the physical construction of places and the organization of activities within them.

The most significant signal of the disappearance of the sense of place is the loss of appeal of cities to people's activities, the neglect of public space, and the lack of sense of identity and participation in the society. In developed countries, the mainstream understanding of urban design has become more and more inclined to the construction process as place identity [19]. Peter Buchanan believes that urban design is essentially about placemaking, and place is not only a special space, but also the activities and events that make the city a place [20]. Among the five evaluation criteria of urban design, "vitality" is the first of the five criteria, which is an important goal of urban design [21]. Gehl believes that the vitality of the urban public space lies in the people in it and their activities. Only the users of space and the activities they participate in are the fundamental factors that determine the vitality of urban public space [10].

Therefore, the accessibility of public space refers not only to physical access, but also social inclusion, which is even more important. It should open to both "people" from different social classes, races, and groups and "different social activities" in accordance with space rights. This is similar to the concept of "publicness" in the field of political philosophy, which is reflected in space, that is, whether the meaning of spiritual and cultural levels expressed by space through the material environment is the most inclusive [22]. Human activity is the fundamental factor of vitality. A high-quality urban space can connect

activities, images, and forms to foster a sense of place. The important goal of urban design is to restore or rebuild the harmonious relationship between people and place, and turn the public space into a public realm [13].

#### *2.2. Typologies and Morphology of Public Space*

According to Conzen, urban morphology is the study of the structural forms of cities, which aims to explain the spatial layout and composition of urban organizations and public spaces. He possed material characteristics and symbolic meanings, the power to create, expand, diversify, and change forms [23]. Scholars such as Sadeghi and Abarkan argue that a systematic interpretation of the forms and types of man-made urban fabric developed over time and combined with social change can provide a good understanding of the present society and the potential needs for future development [24,25]. Yang et al. argue that urban form explains to some extent the logic of urban functioning, and that spatialtypology interacts with social conditions to shape public life through relevant architectural and urban design theories [26]. These different typological combinations of public spaces are thus not only the emerging "urban public sphere" but also the "connecting parts" of a space [26]. Representing and describing urban organization according to its formative processes is the basis for understanding and planning the evolution of the urban realm [7].

The research exploring the association between public identity, participation, and the shape of urban public space is broadly divided into the following assessment perspectives. First is the individual types of public space itself. Lynch's monograph classifies spaces into perceptible landmark, node, path, distinct, and edge, based on psychological cognitive research [21]. Yılmaz Çakmak classifies public spaces into point-like space-activity and linear space-passage, based on spatial morphology and activity patterns [27]. Benjamin classifies public spaces into point-like space-activity and linear space-passage based on urban morphology. He discussed the historical development process by classifying spaces into food production areas, parks and gardens, recreational spaces, plazas, streets, transport facilities, and incidental spaces, based on their function of use [28]. In this way, he discusses the reproduction process of the daily life of the residents and the types of space during the historical development.

Next, the physical environment is characterized by a combination of public space forms, which are mainly focused on accessibility and permeability. The accessibility perspective, which can be said to be a common and valued perspective in recent years, was assessed by Zhou, Lotfi, and Le Texier et al. as the value of accessibility for residents' participation in public green spaces, which is driven by the intrinsic need for recreation and accessibility is strongly related to residents' need for participation [11,29,30]. A. Yavuz's study shows that permeability has positive implications for increasing public participation [31].

Individual-based cognition is also an important perspective in the study of public space. Öztürk, Thirumaran et al. showed that the visual features during movement can attract interest and thus generate memory and identity [32,33]. Mahendra showed that cultural features and symbols have a strong role in visual attraction and can convey the place identity through visuals by extracting elements from the urban forms of India [34]. The lack of visual and physical coherence may lead to a more utilitarian organization, with the concept of function gradually shifting from the external to the internal space. A building often becomes more like a separate part, separated from its environment.

#### *2.3. Methods for Identify Publicness in Urban Space*

In addition to social observation and agent-based simulation methods [35,36], the current quantitative analysis methods for urban research are divided into two main categories, one of which is spatial syntactic analysis based on network analysis [37,38]. The other is spatial clustering analysis, based on location [39,40]. Space Syntax is a typical method which was proposed by Bill Hillier based on social network theory and graph theory, where paths are considered as objects to be brought into the network for analysis [41]. Thus, the interpretation in social network theory converts the closeness centrality of the time

required for information to be sequentially propagated from each node to all other nodes into paths that represent the time for each node that constitutes the path network to reach the location of the remaining nodes, i.e., path reachability is expressed as integration in Space Syntax [37,41]. The betweenness centrality, which is interpreted in social network theory as the number of times a node acts as a bridge to the shortest path between two other nodes, is represented in Space Syntax as choice, which refers to the location through which a person has a high probability of reaching other paths from the current path [37,41]. This network-based analysis has a very significant role in studying cities. Zeng et al. utilized Space Syntax to conclude that accessibility is a key element of connection types, which can connect above ground, underground, indoors, and outdoors, forming spatial sequences with different thematic and morphological characteristics [42,43]. Nilufar utilized Space Syntax to analyze the accessibility of Dhaka city during its evolution and explored the process of urban core change, while pointing out that the locational advantages of areas with high accessibility are significant [44].

Location-based spatial clustering analysis, mainly based on the distribution distance between locations as weights, generates individual clusters to explain their possible causes according to the phenomenon of heterogeneity in their distribution [39,40,45]. Telega used kernel density estimation for urban functional areas to obtain the potential value of different areas for walking in combination with the accessibility of paths [46]. Delso et al. combined network analysis with kernel density analysis to assess the possibility of pedestrian obstacle impacts to improve walking conditions and accessibility [39].

#### **3. Background: The Evolution of Spatial Organization in Xinjiekou**

The site of this research is located in the northeast area of Xinjiekou which is adjacent to Zhongshan Road to the west, Hongwu North Road to the east, Zhongshan East Road to the south, and Changjiang Road to the north (Figure 1). The formation of the northeast area spatial pattern of Xinjiekou is directly related to the opening of Zhongshan Road and Zhongshan East Road in 1928. Guofu road to the north (now Changjiang Road), Hongwu North Road to the east, and the important internal channels such as Tangfang bridge and Qingshi street have always been there since the beginning of development. By 1929, the fundamental layout of the northeast area was established, basically tallying with the time when the pattern of the Xinjiekou area was formed.

The name "Xinjiekou" can be traced back to the early Ming Dynasty (1368s). In 1928, Zhongshan Road, Zhongshan East Road, Zhongshan South Road, and Hanzhong Road were built before Dr. Sun Yat-sen's coffin was brought back to be buried at the mausoleum on Zijin Mountain. At the intersection of these roads, a ring square with a statue of Sun Yat-sen was set up. It was then that the basic pattern of Xinjiekou area took shape. In its 90 years of expansion and development, the area has also been affected by the structural changes of urban morphology (Figure 2).

After the outbreak of the World War II in 1937, urban development entered a state of chaos. Xinjiekou once became an important window for political promotion. From the 1950s to 1980s, the fragmented retail businesses in Xinjiekou were replaced by large department stores, so that a neat and uniform business pattern was initially formed. The evolution of the spatial structure of Xinjiekou at this stage was characterized by the outward expansion of the built-up area. Building density was enhanced from the street to the interior of the parcel, and the proportion of commercial business land grew significantly.

In the 1990s, China began rapid urbanization. The urban fabric of Xinjiekou area radiated to the surrounding area, which marked the subdivision of business office functions and the emergence of the central business district.

**Figure 1.** The site is located in north of Xinjiekou which is the central city of Nanjing, Jiangsu Province, China. (Left, China Standard Map from http://bzdt.ch.mnr.gov.cn/, accessed on 25 June 2022).

**Figure 2.** The spatial structure of Xinjiekou evolved and the public participation of the residents. In the 1920s–1946 it was mainly as a square for walking through, in 1964–1990s the public participation opportunities of the residents increased, and the participation mode was mainly political activities and leisure activities. In the 2000s onwards, motor vehicles occupied the main position and the public participation capacity decreased. (History photos from http://dfz.nanjing.gov.cn/, accessed on 18 April 2021.)

Figure 2 shows the historical changes in the spatial organization of Xinjiekou and its surroundings between 1920s and 2020s. It gradually shifted from a plaza with a high level of public participation to a plaza–pedestrian public space with mainly motorized traffic and indirect participation of the residents, and eventually to a public space for walking only. The residents' participation and identity gradually decreased. The space is gradually segmented and the sense of belonging is gradually fragmented.

#### **4. Research Methodology**

As previously described, the public nature of cities is characterized by public participation in everyday life, thus accessibility is theoretically beneficial in shaping the public nature of space and the typology of space is also seen as an effective way to increase the participation of residents [11,29,30]. Therefore, the objective of this study is to obtain correlations between the distribution of people's activities in a site and the organization of accessibility and space-typologies [28].

In order to understand the site more comprehensively, this study is divided into two steps (Figure 3). First, we will analyze the evolution of the spatial structure of the site in order to obtain the spatial accessibility, and this step aims to obtain the potential of public participation at a larger scale. If the sites selected for this study have a high accessibility in the urban evolution and a high and low distribution of task activities within the sites, then zoning the different spatial types and comparing them will be meaningful. The transition from the scale of the context in which the site is located to the site and the local scale of the site will allow us to better understand the association between spatial form and publicness (Table 1).

**Figure 3.** The publicness of public space in Xinjiekou area and the spatial morphology correlation research route.

**Table 1.** Indicators and definitions of spatial publicness research.

The second step is zoning into the site itself. The first step is to observe the scene and map the spatial distribution of people. In this study, observation points were identified and photographed and counted every 50 m along the paths within the site. The more people are active in a certain location, the more publicness is demonstrated. Secondly, we classify the public spaces in the site and analyze their intermediate centrality. The gentle transition of the intermediate centrality of the spatial form indicates good connectivity and a coherent spatial experience for the residents, which is conducive to the creation of publicness in the space. Secondly, the accessibility of different spaces is analyzed to show the potential of residents' participation. Finally, the correlation between the number of public participants and the mediated centrality and accessibility is determined statistically.

Since this project is based on a positive design, we compare accessibility and mediated centrality before and after the design to predict the effect of spatial design on reshaping the publicness of the space. The space and its adjacent spaces will become more coherent and accessible, thus shaping an urban space that has the potential to become a public realm.

In analyzing accessibility and mediated centrality metrics, this study uses Space Syntax. Space Syntax, proposed originally by Professor Hillier, aims to systematically depict the interaction between individuals and their society and environment. The heterogeneity of space and everyday life is described by quantifying the configurational characteristics and scales of urban space [37,41]. The statistical method uses Geographically Weighted Regression (GWR) to determine the degree of fitness and confidence of the variables. GWR, proposed by Professor Fotheringham, can reduce the influence of location distribution on the statistical results and show the correlation characteristics of variables in a local group compared to the traditional ordinary least squares [47,48].

#### **5. Results and Discussion**

#### *5.1. Context of the Spatial Organization of Xinjiekou*

As mentioned earlier, the spatial organization of Xinjiekou underwent a politicized and commercialized transformation during the historical development of the city's central district. The newly opened road in 1928 gradually became the center of the entire city's central district. The results of the analysis according to Space Syntax reflect the gradual strengthening of the INTEGRATION of the northeast part of Xinjiekou from 1898 to 1933 to 1945 (Figures 2 and 4). This indicates that the physical accessibility of the Xinjiekou area gradually improved, allowing more access for city residents. At the same time, the location of Xinjiekou has maintained a high accessibility from 1945 to 2012, and the historical images in Figure 4 show that Xinjiekou has had a high level of public participation for a long time. The square of Xinjiekou has also become an important public space for political demonstrations and rallies.

**Figure 4.** The evolution of the spatial structure of Xinjiekou on its spatial accessibility from 1898s to 2012s. (History map from http://dfz.nanjing.gov.cn/, accessed on 10 July 2022, digital map from https://www.google.com.hk/maps, accessed on 15 July 2022).

The accessibility of Xinjiekou and its context indicates the possibility of public participation on a regional scale, i.e., in a location that is easily accessible to the public and where daily activities are frequent. However, the scale of this macro-analysis is too large, ignoring the impact of local space shaping on the individual participants and on the cognition of human scale. It is meaningful to discuss the public participation in Xinjiekou base in this context, because the high accessibility and high participation attributes make this study representative of a post-zoning analysis of its spatial characteristics [4,11].

#### *5.2. Correlation of Public Participation with Choice and Integration in the Northeast Area of Xinjiekou*

As mentioned in the research methodology, we first made a comprehensive observation of the site after zoning to the Xinjiekou area. As shown in the Figures 5 and 6, a total of 146 observation points were selected for this study, and the location and number of people passing by were photographed to record their activities. The original space of Xinjiekou after the simple renovation in 2012 was a mixed-use pattern of commercial building space, residential buildings, and traditional residential houses.

In this study, we divided public spaces into two categories. The first category was path, a type of space in which the public participates by walking, with little stopping, and the width of the path is within 3–10 m and the length is between 50–500 m. The location is usually on one side of the building with a small amount of landscape. The second type of space is node, where the public participates in more diverse ways, socializing, stopping, resting, etc. Its size is 50 m × 50 m or more, which can create plazas, gardens, etc.

By observation, the spatial ratio is partially imbalanced, with commercial building spaces large and orderly, and residential buildings small and scattered, and the spatial sequence is in a fractured state. The commercial and residential spaces are self-contained and lack integrity. The public space connecting the different physical spaces is reduced to a non-architectural public domain, where residents rarely stop, walk, and move around, and have a low sense of belonging. From the diagram, we can see that the distribution of characters is mainly located in the central paths of the site and at the entrances and exits of some of the plots. At the same time the distribution of locations of higher public participation is disorderly and discontinuous.

At the same time, we analyzed the integration and choice of the site (Figure 6). According to the analysis results, we can see that the paths with higher choice are located around the base, and the overall choice of the site is low, while the center is special as a bridge

between residential and commercial areas with higher choice. The accessibility analysis results show that, except for the residential paths with lower accessibility, the accessibility of the rest of the commercial locations is more homogeneous.

**Figure 5.** Top left, the distribution of public space types in the site with the distribution of building functions. Upper middle, index of actual photos of the site. Upper right, location of observation points for recording the number of public participants. Bottom, actual photos of the site.

**Figure 6.** On the left, the distribution of the number of residents' public participation. In the middle, the analysis of the spatial syntax on the choice value of urban morphology. On the right, the spatial syntax analysis of the integration value of urban form.

Based on this, we used GWR to analyze the correlation between the number of public participants, choice, and integration (Table 2 and Figure 7). At the same time, we can see that the R2 of the model fit is 0.392, and the fit of the model constructed by choice, integration, and the number of participants is relatively average. The number of effective parameters (ENP) is 13.801, which indicates that there are still variables not considered in the model except for choice and integration. And the p value of the variable is less than 0.01, indicating that the analysis result is very important. Meanwhile, the overall effect of choice on public participation is positively correlated, while the overall effect of integration on public participation is negatively correlated, which is different from what we usually think. This (Figures 6 and 7) suggests that in small-scale neighborhoods, the coherence of the paths will drive public participation, while the accessibility of the paths does not drive residents to participate in the activities on the site, thus making the public space lose its publicness.

**Table 2.** Diagnostic information of the statistical model of the number of public participants based on GWR and CHOICE, INTEGRATION.

**Figure 7.** Distribution of the correlation between public participation, and choice and integration.

In addition, among the spatial types of paths (Figures 6 and 7), the overall performance is that high accessibility presents a positive correlation with public participation when the paths with higher choices are connected. Accessibility presents a negative correlation with public participation when the paths are not connected. In the spatial type of node, choice– integration correlation is consistent with node correlation, so the strategy to promote public participation in the spatial type of node should be adopted to increase choice–integration in parallel.

#### *5.3. Pedestrian Flow Design Strategy for Xinjiekou Public Space*

The design reconfigured the pedestrian flow based on the correlation between public participation and choice and integration as summarized by the site. We retraced the choice and integration of traffic on the site from 2005 to 2012, and the overall spatial organization remained unchanged except for the closure of pedestrian access around the road intersection (Figure 8). The choice of pedestrian flow in the site is locally enhanced and the paths of high choice are not coherent, and the accessibility of pedestrian flow in the site has been improved. Based on the previous analysis, we can understand that in this case the public participation of the residents will be weakened, thus weakening the public nature of the public space of the site.

**Figure 8.** Spatial accessibility and choice analysis, from 2005 to 2012 to 2019.

Based on the above analysis, we can understand that the public participation of the residents will be weakened in this case, thus weakening the public space of the site. According to the analysis results, we can find that the choice-value of the pedestrian flow is higher and the paths with high choice-value are more coherent. At the same time, the integration of these paths has also improved significantly. Based on the previous analysis, we believe that the public space in this case has increased the potential for residents to participate in the site, thus increasing the public nature of the public space.

In conclusion, this study developed an urban design strategy based on field observations and quantitative analysis of Space Syntax. We created the publicness of public spaces in urban centers by developing situational strategies based on an understanding of the evolutionary characteristics of urban organization. In addition to satisfying functional use, it increases the possibility of interaction and communication between residents.

#### **6. Conclusions**

Many of China's urban design strategies tend to position themselves in terms of commercial value and neglect the creation of the human element of urban life. This has led to increasing social separation and a loss of the sense of urban belonging. This has had a negative impact on both the city and its residents. We believe that full awareness of the potential needs of society and future development is an important prerequisite for design. In this study, we also argue Sadeghi, Abarkan, and Yang et al. that urban form explains to some extent the logic of urban function, that spatial-typology interacts with social conditions and can provide for the shaping of public life and publicness of space [24,25,41].

In this study, we first examined Xinjiekou in its contexts through the Space Syntax method to verify whether the smaller area selected for the study was representative of the study framework. Second, through the quantitative analysis of Space Syntax and social observations, it was determined that public participation tends to choose walking paths with a higher number of shortest paths through a location and that these locations are interconnected. Under this condition, accessibility has a catalytic effect on public participation. If these high-choice walking paths are not connected, residents are less likely to choose such areas for their activities. Later in the design, we organized the pedestrian circulation in the site based on the findings of the analysis, thus contextualizing, and enhancing, the possibilities and potential for future residents to participate in the daily life of the public space.

In this paper, the study provides a pre-feasibility hypothesis for the design that combines the correlation between the quantitative analysis of spatial morphology and the spatial distribution characteristics of social phenomena to construct a public participationoriented cognitive framework of urban morphology. The method not only effectively delineates the interaction process of minute forms to the overall human–spatial, but also has a statistical-based verification process with high interpretability and confidence. The designs generated based on this framework are expected to facilitate the evolution of public space into the public realm upon completion. We hope that this combination of observation and quantitative analysis will validate and optimize design theory in anticipation of enhanced placemaking in urban public spaces to improve the lives of residents.

The current study has the following shortcomings that we hope to explore in future research.

**Author Contributions:** Conceptualization, M.P.; methodology, M.P.; software, Y.S.; investigation, M.P., Y.S. and Q.J.; resources, Q.Z.; data curation, Q.Z. and Y.L.; writing—original draft preparation, M.P.; writing—review and editing, M.P.; visualization, M.P.; supervision, Q.Z. and Y.L.; project administration, Q.Z. and Y.L.; funding acquisition, Q.Z. and Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Architects and Engineers Associates of Southeast University (No. 51778123).

**Institutional Review Board Statement:** Since this study did not involve personal privacy nor questionnaires, etc., and only the authors' own observations were used as data collection, ethical approval was not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Structural Landmark Salience Computation in Compact Urban Districts with 3D Node-Landmark Grid Analysis Model: A Case Study on Two Sample Districts in Changsha, China**

**Yang Guo 1,2, Xijun Hu 1,2,\* and Jia Tang 1,2**

**Abstract:** Mastering the relationship between urban landmarks and urban space morphology in urban planning, landscape planning, and architectural design helps maintain the intelligibility of compact urban districts. The objective of the present study was to numerically determine the structural salience of various landmarks in an urban environment and use it to interpret the intelligibility of the city. Combining the measurement method of 3D visibility and the related principles of space syntax, this study develops a new 3D Node–Landmark Grid Analysis Model (3D NL GAM) for structural salience computation of urban landmarks. In this study, a numerical approach is used to construct a 3D simulation model. Firstly, the visibility of each decision node to landmarks in an urban environment, using a 3D digital model, is measured using the 3D isovist component of Rhinoceros and Grasshopper software. Secondly, links among wayfinding decision nodes and landmarks are established to form a 3D NL GAM. The normalized angular integration of decision nodes and the normalized angular choice of landmarks are computed using the principle of space syntax. Thirdly, the structural salience of landmarks is determined with a function of landmark visibility, spatial properties of landmarks, and wayfinding decision nodes. Finally, a case study was carried out by using a 3D NL GAM to analyze three types of urban areas located in Changsha. The results indicated that large-scale natural landscapes have a higher structural salience among the types of landmarks. The structural salience of architectural landmarks in the combined spatial form of combining tall and low building groups has a clear advantage over the form dominated by high-rise building groups. Raising the height of landmark buildings can modify the structure of the grid analysis model and improve the people aggregation of urban space. The 3D NL GAM can quantify the spatial properties and landmark structural salience of a city and can effectively assist in the evaluation of the intelligibility of built or future urban environments.

**Keywords:** structural salience; landmark visibility; space syntax; 3D isovist; node–landmark grid analysis model

#### **1. Introduction**

Compact cities are an important means of sustainable development and are conducive to the intensification of urban resources, but they also present higher requirements for urban planning, landscape planning, and architectural design [1–3]. UN-Habitat advocates moderately compact and high-density cities, and suggests that the population density of future urban planning should be greater than 150 people per hectare [4]. This has led to higher population densities, mixed-function land use, more centralized public facilities, compact residential built communities within a given urban area, and the development of vertical space. "Intelligibility" plays a crucial role in the quality of urban space [5].

**Citation:** Guo, Y.; Hu, X.; Tang, J. Structural Landmark Salience Computation in Compact Urban Districts with 3D Node-Landmark Grid Analysis Model: A Case Study on Two Sample Districts in Changsha, China. *Buildings* **2023**, *13*, 1024. https://doi.org/10.3390/ buildings13041024

Academic Editors: Michael J Ostwald and Ju Hyun Lee

Received: 24 February 2023 Revised: 4 April 2023 Accepted: 11 April 2023 Published: 13 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

However, high density and high-rise clusters in compact cities tend to block views in urban spaces, which affects the permeability of urban spaces and undermines urban intelligibility. Therefore, intelligibility in urban environments has become an important topic in urban design for compact cities. In order to ensure the quality of urban space and enhance urban intelligibility, urban designers need to think about the following questions: (1) how to obtain comfortable perception density in urban street space [6–8], (2) ensuring the synergy of landmarks, (3) how to garner the necessary landscape resources, (4) the synergy between specific location and the overall environment [9,10], and (5) other urban image elements in relation to urban residents' activities. Among them, where clarifying the structural relationship of landmarks in compact urban areas, accurately describing the structural salience of landmarks in a compact urban environment is a key issue for enhancing urban intelligibility.

Urban intelligibility is the original goal of urban imagery in terms of how people orient themselves and navigate the city [5,11]. People's actions are governed by their visual perception and influenced by the characteristics of urban imagery. In essence, urban imagery describes the structural salience of the city's elements in relation to each other [9,12]. The integration of the five elements of urban imagery reflects the behavior of people moving in the city, i.e., for a person moving in an urban area, this is the process of perceiving and acting in this area, with boundaries determining the scope of analysis, paths as channels, nodes as decision points for action, and landmarks as references. Therefore, the structural salience of landmarks is based on the integration of nodes, paths, areas, and boundary forms, expressing the role of landmarks in the spatial structure of urban areas and their relationship with other elements [13].

The structural salience of landmarks has substantial implications for wayfinding and navigation in cities [13–16]. Caduff et al. generated landmark route navigation using a landmark saliency-weighted path algorithm and compared landmark routes with shortestdistance routes, which demonstrates the advantage of landmark routes for navigating route selection in most cases [17,18]. Simulation analysis agent-based modeling has shown [19] that the route selection model containing landmarks provides directional guidance and environmental characteristic variability, thus expressing more urban information. That is, the urban information conveyed by landmarks is related to the visibility of landmarks, and the analysis of landmarks in urban areas should consider not only neighboring but also distant landmarks. In terms of the location relationship between landmarks and pathfinders, landmarks are usually classified into global and local landmarks [15,20]. Global landmarks refer to landmarks that can be seen from a large distance, and a global reference system, such as the efficacy of a compass in landmark routes. Local landmarks can only be seen from a small distance; they are only visible in a limited area and only from a specific method [15,20]. Studies have shown that both local and global landmarks are used for decision making [15,20–22].

However, there has been a lack of quantitative characterization metrics and scientific analytical methods to describe the structural salience of landmarks in urban spaces [23]. In terms of landmarks and human wayfinding needs, landmarks are defined as "geographic objects that constitute human mental representations of space" [24], and landmark representations are the result of the local integration of visual, motor, and spatial information [25]. Klippel and Winter determined the structural salience of landmarks from the semantic expression of the positional relationship between landmarks and navigation routes. This is a kind of qualitative research method focused on navigation route arrangement [26], where it is difficult to form a difference comparison for the structural salience of landmarks. Winter also proposed an extension of the advance visibility assumption, in which landmarks can be identified early along the roadway with valid landmark structural saliency [27], but does not provide a measure of visibility in complex 3D urban environments and miscellaneous road grids. Therefore, the structural salience of landmarks in urban environments is a unique property of the cognitive and physiological trilateral relationship between the landmark feature itself, its surroundings, and the observer's perspective [11,14,28].

Space syntax theory provides methods for measuring spatial node accessibility and directional guidance in cities [29]. Claramunt and Winter adopted the method of mathematical graph theory and space syntax to analyze the saliency of regions, boundaries, streets, and nodes and formed a structural model of urban element recognition [12,30]. It has been shown that the principles of space syntax are suitable for the structural description of urban spatial forms, and quantitative analytical results can be obtained. However, this analytical approach does not deal with distant landmarks on the one hand, nor does it consider the impact of the visibility of nearby landmarks on the urban structure model. Therefore, based on the two-dimensional limitation of urban grids, space syntax cannot adequately describe landmarks [31]. Enhancements based on 3D isovist generation techniques have promoted the development of spatial syntactic analysis toward 3D spatial analysis. These have been realized in spatial analyses regarding the measurement of urban spatial openness [32], the spatial morphological characteristics [33], as well as visual permeability measurements [34]. Detailed representations of spatial visibilities have been implemented in these studies, but they have not been synchronously implemented for the visibility of a particular object.

The above literature gives an important insight that if a 2D grid analysis model for an urban network can be developed into a 3D grid analysis model, then the objective of accurately quantifying the structural saliency of landmarks is possible. Two conditions must be satisfied to achieve this objective. First, visibility measurements of urban landmarks are implemented for all the wayfinding decision nodes in a 3D environment. Second, the spatial properties of landmark locations and the spatial properties of wayfinding decision nodes are implemented for directional guidance and spatial accessibility. With the improvement of computer hardware and software technology, the current measurement technology for both 3D digital models of cities and visibility data of spatial places has been significantly improved [35,36]. Meanwhile, there is a mature research paradigm for analyzing the relationship between urban spaces using space syntax [37,38]. In order to achieve these two fundamental conditions, the next section is devoted to a literature review on the construction of a grid analysis model based on space syntax, and the presentation of a 3D isovist.

Based on the literature and analysis, this study develops a 3D Node–Landmark Grid Analysis Model (3D NL GAM) to incorporate the 3D visibility data of landmarks into the spatial syntactic analysis model. In the context of compact cities, this paper aimed to clarify the integrated relationship between landmarks and textural features in urban areas. In particular, the relationship between natural landscape resources, street grids, the combined forms of different architectural groups, and the landmark's own form was examined and used to guide the evaluation of urban intelligibility and the urban design.

#### **2. Literature Review**

#### *2.1. Grid Analysis Model Based on the Principle of Space Syntax*

Space syntax is a scientific reconstruction of how people interact with their environment. This helps to understand the relationship between humans and their environment by creating a model that connects real spaces with human cognitive structures through logical spaces (employing signs, symbols and representations to create schematized multidimensional spaces) [39]. After more than 50 years of development, space syntax has become a paradigm for spatial research, and is linked to other disciplines or research fields [40]. Using the principles of space syntax, the city is understood as a network formed by space, thus establishing a link between human behavior and network science in the urban environment [41,42]. The analytical method of space syntax can be used to analyze the urban street networks, functional layouts, data on urban points of interest, as well as the structure of urban centers and other mobile features of human agglomeration and dispersion in the urban environment [43–46].

Traditional grid analysis models for space syntax include convex map analysis [47], axial map analysis [48], and Visibility Graph Analysis (VGA) [49,50]. An urban network is used to abstractly express the location, connectivity relationships, and the structure of the

city's nodal spaces in a region-wide system, as well as to form a corresponding structural form with various types of analytical models [51–53]. The approach to grid construction is consistent with the principles of mathematical graph theory, i.e., a grid graph is composed of nodes and lines, where nodes correspond to the vertices of the graph, in mathematical graph theory, and lines correspond to the edges of the graph [30]. Thus, there are two core tasks for constructing an analytical model:

Traditionally, there are three ways to construct a grid. The first way is to consider streets as spatial nodes, with axes acting as the logical space of streets and intersections between the streets acting as connecting edges, which is used to calculate the motion properties of street space in the form of an axial analysis map [29]. The second way treats street intersections and turning points as spatial nodes and street segments as connecting edges, which is then used to analyze the spatial movement properties of street intersections in the form of convex analysis maps [12,30]. The third way belongs to VGA. First, isovist is employed to express a field of view; an isovist is the set of all visible objects of a viewpoint formed according to set viewpoint parameters [54]. Essentially, an isovist is 3D, and only a cross-section containing the viewpoint is taken in the viewable analysis model construction, i.e., a 2D isovist [54,55]. The grid spacing as a raster is filled in the view plane of the urban space, and a cell of the raster represents a node, which is the logical space of the grid. The connectivity between nodes is determined by applying the visibility principle of isovists to determine the accessibility between nodes. All nodes contained in the 2D isovist generated by a node are neighbors of that node, and the domains are connected to each other so that they can be used to analyze the spatial mobility properties of all nodes in the urban planar space.

The essence of grid analysis model construction is to reflect the relationship between spaces in as simple and accurate way as possible [56]. Batty has compared the above three grid analysis models, where the setting of the axis and the convex is set manually with some subjectivity, while the raster can be automatically generated after setting the accuracy [56]. Based on the VGA analysis model, it can automatically generate more axial analysis maps consisting of axes, while the convex itself is a subset of the raster plane, and the removal of some unwanted cells becomes the convex analysis map [56]. Therefore, the second method describe can be understood as a simplified version of the traditional VGA graph, where the intersections are located on the same linear street and close elevations are all neighborhood nodes. That is, n intersections on the same linear road are neighbors of each other, and the grid is a graph with a combination of n vertices and *C*2 *n* bar edges.

Integration and choice express the motion properties of spatial nodes. The integration originates from the concept of node closeness centrality in graph theory, i.e., the smaller the cumulative value of the distance from the point to all other points, the more it indicates that the node is close to the center in the system [12,30]. The integration is expressed by the space syntax as the reciprocal of the depth of a spatial node relative to other spaces; therefore, the integration reflects the structural relationship of a node with others [57]. The higher the integration of a node, the better the accessibility, and thus the easier it is for a moving population to cluster or stay at that spatial node. This choice originates from the probability of a node intervening in the shortest path between two nodes in graph theory, and is referred to, in graphs, as betweenness centrality [12]. Space syntax theory uses choice to analyze the degree of movement through a node and is a powerful tool for predicting the potential of pedestrian and vehicular movement. Spaces with high global choice records lie on the shortest paths from all origins to all destinations [29]. The choice usually describes the movement through a place rather than possession or a stay.