LiDAR

Technical Aspects

LiDAR systems are operated in either scanning or profiling mode. Profiling-mode LiDAR sensors emit laser pulses in a single direction, while scanning mode systems sweep laser pulses from side to side as they exit the sensor. Scanning LiDAR systems, when placed on an airborne platform and integrated with a global positioning system (GPS) receiver and inertial measurement unit (IMU), are able to accurately map large areas of the earth's surface at high resolutions. Each recorded laser pulse reflection collected is typically output in some Cartesian (x, y, z) coordinate system, which can be transformed into a real-world coordinate system.

Airborne LiDAR mapping sensors can emit and acquire laser pulse return data at very-high-pulse repetition frequencies (PRF). Currently available technology can operate at over 100,000 pulses per second, fly at up to 4,000 m above the ground surface, scan at up to +/–30 degrees from nadir, and map several hundreds of square kilometers every hour. The resulting x, y, and z point coordinate densities can range from as much as several meters apart down to tens of centimeters apart. This can be considered analogous to setting up a surveying total station and taking x-, y-, and z-coordinate readings every, say, 1 m × 1 m over the landscape. When several LiDAR surveys are conducted over the same area during an extended period of time, temporal LiDAR records provide data for estimation of changes in features such as glacier volumes, landslide movement, and ongoing erosion processes.

Laser pulses emitted from airborne LiDAR systems reflect from objects both on and above the ground surface, including vegetation, buildings, bridges, and so on. Any emitted pulse that encounters multiple reflection surfaces as it travels toward the ground is “split” into as many “returns” as there are reflecting surfaces. Those returns containing the most reflected energy (i.e., reflecting from the largest or most highly reflective surface areas) will be observed and recorded at the sensor, while the weakest returns will usually not be recorded. Most sensors will allow this laser pulse “intensity” to be recorded along with the positional information. This multiple-return capability distinguishes LiDAR from many other remote-sensing technologies, which are often unable to penetrate through dense vegetation canopies to the ground surface.

Due to this multiple-return and foliage penetration capability, LiDAR data obtained over vegetated environments can readily be filtered to separate ground and nonground returns, thus revealing the true ground surface topography. Frequently, LiDAR data sets are used to create irregular network digital terrain models (DTM), raster digital elevation models (DEM) of the filtered ground surface, raster digital surface models (DSM) of the unfiltered all return data, and raster canopy height models (CHM).

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