What Are LIDAR Data?

May 15, 2008 by airianto  
Filed under Geomatics Science, LIDAR


Light Detection and Ranging (LIDAR) is a remote sensing system used to collect topographic data. This technology is being used by the National Oceanic and Atmospheric Administration (NOAA) and NASA scientists to document topographic changes along shorelines. These data are collected with aircraft-mounted lasers capable of recording elevation measurements at a rate of 2,000 to 5,000 pulses per second and have a vertical precision of 15 centimeters (6 inches). After a baseline data set has been created, follow-up flights can be used to detect shoreline changes.

How LIDAR Data Are Collected

For the South Carolina project, a LIDAR sensor was mounted on-board a NOAA DeHavilland Twin Otter aircraft pictured below. Once in flight, the aircraft travels over the beach at approximately 60 meters per second. During the flight, the LIDAR sensor pulses a narrow, high frequency laser beam toward the earth through a port opening in the bottom of the aircraft’s fuselage. The LIDAR sensor records the time difference between the emission of the laser beam and the return of the reflected laser signal to the aircraft.

NOAA Twin Otter Aircraft

NOAA Twin Otter Aircraft - www.csc.noaa.gov

The LIDAR transceiver is rigidly fastened to the aircraft and does not move. However, a scan mirror assembly is mounted beneath the transceiver. A 45-degree folding mirror reflects the laser pulses onto a moving mirror which directs the laser pulses to the earth. The reflected laser light from the ground follows the reverse optical path and is directed into a small Cassegrainian telescope. The moving mirror produces a conical sampling pattern beneath the aircraft over a 30-degree wide swath, thus permitting the collection of topographic information over a strip approximately 300 meters (about 1000 feet) in width from the nominal 600 meter (2000 feet) data collection altitude. For an animated display of the data collection process, click here.

The Scan Width of the LIDAR Sensors

Illustration of How the LIDAR Sensing Instrument Captures Elevation Points - www.csc.noaa.gov

Illustration of How the LIDAR Sensing Instrument Captures Elevation Points.

The LIDAR instruments only collect elevation data. To make these data spatially relevant, the positions of the data points must be known. A high-precision global positioning system (GPS) antenna is mounted on the upper aircraft fuselage. As the LIDAR sensor collects data points, the location of the data are simultaneously recorded by the GPS sensor. After the flight, the data are downloaded and processed using specially designed computer software. The end product is accurate, geographically registered longitude, latitude, and elevation (x,y,z) positions for every data point. These “x,y,z” data points allow the generation of a digital elevation model (DEM) of the ground surface.

LIDAR data sets on this CD-ROM cover an area from the low water line to the landward base of the sand dunes. Flights are planned to maximize the number of elevation points collected at the lowest tide for the largest area possible. The aircraft flight path is always parallel to the beach. Four passes are flown over each section of the beach. Two of these passes are flown so the center of the swath is over the sand/water interface. The other two passes are flown over the center of the sand/development interface.

Flights generally last four hours. Weather conditions must be monitored. The flights cannot be flown during times of rain or fog as the water vapor in the air could cause the laser beams to scatter and give a false reading. Additionally, the plane cannot fly during times of high winds as the returned laser pulse will not be recorded correctly.

Interpreting LIDAR Elevation Maps

In remote sensing, false color images such as LIDAR elevation maps are common. They serve as an effective means for visualizing data. The term “false color” refers to the fact that these images are not photographs. Rather, they are digital images in which each image pixel represents a data point that is colored according to its value. The purpose of this section is to aid users in interpreting false color images.

LIDAR beach mapping data are composed of elevation measurements of the beach surface and are acquired through aerial topographic surveys. The file format used to capture and store LIDAR data is a simple text file and referred to as “x,y,z,” where x is longitude, y is latitude, and z is elevation. Using the elevation “points,” LIDAR data may be used to create detailed topographic beach maps.

In the three images shown below, the legend in the bottom right corner of the image has a range of numbers from -3 meters to +5 meters. The numbers indicate the relationship between the colors on the legend and the elevations depicted on the map. For example, in the Huntington Beach map, the deep blue color represents land approximately at sea level or zero elevation. The cyan (light blue) features, like the jetty, represent elevations around 1 meter, or about 3 feet above sea level.

Examples of Different Elevation Data Sets

Lidar Data - www.csc.noaa.gov

LIDAR data become easier to interpret when examined in conjunction with additional data such as aerial photography. In the example below a LIDAR elevation map is compared with an orthophotograph. This small area on Kiawah Island provides a variety of interesting features. Comparing the orthophoto to the LIDAR data it becomes easier to identify features such as houses, roads, the vegetated dune area, and irrigation ponds.

Lidar elevation map - www.csc.noaa.gov

Along the South Carolina coast, beach features tend to be less than 5 meters (16 feet). As a result, the scale of the color bar was chosen to highlight relatively narrow variations in elevation. This legend can be readily viewed in the PDF maps located in the pdf/islands directory on this CD-ROM. Addtionally, this legend has been provided for use in ArcView and is located at: data/lidar/avelev.shp.

In this second example, an additional vector base map was overlaid on both the orthophoto and LIDAR elevation map. The base map, created in 1993, includes digitized building footprints, dune walkovers, and roads. A detailed base map can assist in confirming features detected by LIDAR elevation measurements. For example, when houses are surrounded by tall vegetation, LIDAR elevation data do not distinguish between roof top and tree top. Without the vector base map, it would be very difficult to determine boundaries between roofs and trees. Often ancillary data do not provide sufficient detail or are not available. In these cases, the user must obtain ground reference information using either local knowledge or by visiting the area to accurately confirm landmarks.

Example of How Vector Data Can be Useful in Identifying Features in LIDAR Data

Users can also view LIDAR data by creating a plot or profile of the data. In the profile below the beach features including the dune crest, beach face, and the water line can be identified. Users that have the add-on ArcView® Spatial Analyst® module can use the LIDAR Data Handler Extension, provided on this CD-ROM, to create similar profiles. For more information about this tool see the Data Tools section.

LIDAR Data Viewed as a Profile - www.csc.noaa.gov

Source : NOAA – http://www.csc.noaa.gov/products/sccoasts/html/tutlid.htm

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