LiDAR Mapping

Light Detection and Ranging (LIDAR) is a remote sensing system used to collect topographic data. This technology is being used to collect data 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).

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.

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.

Flights are planned to maximize the number of elevation points collected at the lowest tide for the largest area possible. 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.

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