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GIS Theoretical


What is a GIS?


How does a GIS work?

Relating information from different sources

The power of a GIS comes from the ability to relate different information in a spatial context and to reach a conclusion about this relationship. Most of the information we have about our world contains a location reference, placing that information at some point on the globe. When rainfall information is collected, it is important to know where the rainfall is located. This is done by using a location reference system, such as longitude and latitude, and perhaps elevation. Comparing the rainfall information with other information, such as the location of marshes across the landscape, may show that certain marshes receive little rainfall. This fact may indicate that these marshes are likely to dry up, and this inference can help us make the most appropriate decisions about how humans should interact with the marsh. A GIS, therefore, can reveal important new information that leads to better decisionmaking.

Many computer databases that can be directly entered into a GIS are being produced by Federal, State, tribal, and local governments, private companies, academia, and nonprofit organizations. Different kinds of data in map form can be entered into a GIS (figs. 1a, 1b, 1c, 1d, 1e, 1f, and 2). A GIS can also convert existing digital information, which may not yet be in map form, into forms it can recognize and use. For example, digital satellite images can be analyzed to produce a map of digital information about land use and land cover (figs. 3 and 4). Likewise, census or hydrologic tabular data can be converted to a maplike form and serve as layers of thematic information in a GIS (figs. 5 and 6).

A line map showing roads with different colored lines representing types of roads.

Figure 1a. U.S. Geological Survey (USGS) digital line graph (DLG) data of roads.

A line map with various colors representing bodies of water and streams/rivers.

Figure 1b. USGS DLG of rivers.

A map showing contour lines.

Figure 1c. USGS DLG of contour lines (hypsography).

A black and white picture showing DEM shadings representing contours.

Figure 1d. USGS digital elevation (DEM).

A section of a color topographic map.

Figure 1e. USGS scanned, rectified topographic map called a digital raster graphic (DRG).

A black and white picture of map overlaying an aerial photograph.

Figure 1f. USGS digital orthophoto quadrangle (DOQ).

A section of a color geologic map.

Figure 2. USGS geologic map.

A colored modified satellite image.

Figure 3. Landsat 7 satellite image from which land cover information can be derived.

A color picture showing an analysis graphic.

Figure 4. Satellite image data in figure 3 have been analyzed to indicate classes of land uses and cover.

A color picture showing part of a computer screen display.

Figure 5. Part of a census data file containing address information.

A black and white picture of computer screen display of a graph.

Figure 6. Part of a hydrologic data report indicating the discharge and amount of river flow recorded by a particular streamgage that has a known location.

Data capture

How can a GIS use the information in a map? If the data to be used are not already in digital form, that is, in a form the computer can recognize, various techniques can capture the information. Maps can be digitized by hand-tracing with a computer mouse on the screen or on a digitizing tablet to collect the coordinates of features. Electronic scanners can also convert maps to digits (fig. 7). Coordinates from Global Positioning System (GPS) receivers can also be uploaded into a GIS (fig. 8).

A color photograph showing two women operating a scanner and a computer.

Figure 7. Scanning paper maps to produce digital data files for input into a GIS.

A color photograph showing a man sitting in a field working with GPS receiver.

Figure 8. Collecting latitude and longitude coordinates with a Global Positioning System (GPS) receiver.

A GIS can be used to emphasize the spatial relationships among the objects being mapped. While a computer-aided mapping system may represent a road simply as a line, a GIS may also recognize that road as the boundary between wetland and urban development between two census statistical areas.

Data capture—putting the information into the system—involves identifying the objects on the map, their absolute location on the Earth's surface, and their spatial relationships. Software tools that automatically extract features from satellite images or aerial photographs are gradually replacing what has traditionally been a time-consuming capture process. Objects are identified in a series of attribute tables—the "information" part of a GIS. Spatial relationships, such as whether features intersect or whether they are adjacent, are the key to all GIS-based analysis.

Data integration

A GIS makes it possible to link, or integrate, information that is difficult to associate through any other means. Thus, a GIS can use combinations of mapped variables to build and analyze new variables (fig. 9).

A color diagram showing how information is processed.

Figure 9. Data integration is the linking of information in different forms through a GIS.

For example, using GIS technology, it is possible to combine agricultural records with hydrography data to determine which streams will carry certain levels of fertilizer runoff. Agricultural records can indicate how much pesticide has been applied to a parcel of land. By locating these parcels and intersecting them with streams, the GIS can be used to predict the amount of nutrient runoff in each stream. Then as streams converge, the total loads can be calculated downstream where the stream enters a lake.

Projection and registration

A property ownership map might be at a different scale than a soils map. Map information in a GIS must be manipulated so that it registers, or fits, with information gathered from other maps. Before the digital data can be analyzed, they may have to undergo other manipulations—projection conversions, for example—that integrate them into a GIS.

Projection is a fundamental component of mapmaking. A projection is a mathematical means of transferring information from the Earth's three-dimensional, curved surface to a two-dimensional medium—paper or a computer screen. Different projections are used for different types of maps because each projection is particularly appropriate for certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.

Since much of the information in a GIS comes from existing maps, a GIS uses the processing power of the computer to transform digital information, gathered from sources with different projections, to a common projection (figs. 10a and b).

A section of a line map with a color  overlay incorrectly aligned with the lines.

Figure 10a. An elevation image classified from a satellite image of Minnesota exists in a different scale and projection than the lines on the digital file of the State and province boundaries.

A section of a line map with the corrected color overlay.

Figure 10b. The elevation image has been reprojected to match the projection and scale of the State and province boundaries.

Data structures

Can a land use map be related to a satellite image, a timely indicator of land use? Yes, but because digital data are collected and stored in different ways, the two data sources may not be entirely compatible. Therefore, a GIS must be able to convert data from one structure to another.

Satellite image data that have been interpreted by a computer to produce a land use map can be "read into" the GIS in raster format. Raster data files consist of rows of uniform cells coded according to data values. An example is land cover classification (fig. 11). Raster files can be manipulated quickly by the computer, but they are often less detailed and may be less visually appealing than vector data files, which can approximate the appearance of more traditional hand-drafted maps. Vector digital data have been captured as points, lines (a series of point coordinates), or areas (shapes bounded by lines) (fig. 12). An example of data typically held in a vector file would be the property boundaries for a particular housing subdivision.

A grid of numbers representing raster data.

Figure 11. Example of the structure of a raster file.

A color map showing vector data.

Figure 12. Example of the structure of a vector data file.

Data restructuring can be performed by a GIS to convert data between different formats. For example, a GIS can be used to convert a satellite image map to a vector structure by generating lines around all cells with the same classification, while determining the spatial relationships of the cell, such as adjacency or inclusion (fig. 13).

A color section of a raster map.

Figure 13a. Magnified view of the same GIS data file, shown in raster format.

A color diagram of a map in vector format.

Figure 13b. Magnified views of the same GIS data file. converted into vector format.


Data modeling

It is impossible to collect data over every square meter of the Earth's surface. Therefore, samples must be taken at discrete locations. A GIS can be used to depict two- and three-dimensional characteristics of the Earth's surface, subsurface, and atmosphere from points where samples have been collected.

For example, a GIS can quickly generate a map with isolines that indicate the pH of soil from test points (figs. 14 and 15). Such a map can be thought of as a soil pH contour map. Many sophisticated methods can estimate the characteristics of surfaces from a limited number of point measurements. Two- and three-dimensional contour maps created from the surface modeling of sample points from pH measurements can be analyzed together with any other map in a GIS covering the area.

A color map section showing points with numbers.

Figure 14. Points with pH values of oil.

A color map showing points and contours.

Figure 15. Contour map made from soil pH values shown in figure 14.