Lidar and Airborne Laser Scanning

Measurement Method

LiDAR systems use pulses of light amplified by stimulated emission of radiation (laser). The time that lapses between emitting a laser pulse, the reflection of the pulse off of a surface, and its subsequent return to the sensor is used to determine the distance to the surface from the sensor in the equation: Distance = (Rate × Time)/2, where Rate is equal to the speed of light and Time is the aforementioned lapse of time for a pulse to return to the sensor. A global positioning system (GPS) and an inertial measurement unit are used to measure the position of the sensor and attitude (angle) of the aircraft. The position and angle of the aircraft are in turn used with the distance measurement to geographically reference the points where laser pulses reflected off of a surface.

Characteristics of LidaR Sensors

Features of LiDAR sensor systems that may vary include the platform used to carry the sensor, the wavelength of light used for the lasers, whether pulses are discrete or continuous, and scanning characteristics. Most platforms that carry LiDAR systems are either small manned airplanes or helicopters. Some systems are suitable for satellite deployment. Fields such as forestry and surveying use LiDAR technology packaged for in situ, ground-based measurements.

LiDAR sensors often use near-infrared light because it reflects well off of vegetation, in particular, and also strongly off of human-made and other surfaces. Often, LiDAR data are separated into ground and vegetation canopy returns during postprocessing of the data. Applications such as coastal mapping use infrared lasers (which reflect off of the water surface) and green lasers [Page 1779](which travel through water to an extent) in combination to map shallow areas.

Discrete-return LiDAR uses short laser pulses to collect surface information. One pulse may strike multiple surfaces, and each surface it strikes could reflect back a portion of the emitted energy. A multiple-return LiDAR system measures the multiple moments of reflection in the path of one laser, as opposed to recording only the last return. Continuous LiDAR measures the full waveform of each laser pulse by continuously recording the intensity of reflection during the laser's travel.

Profiling LiDAR sends a pulse down at nadir to provide a linear trace, or profile, of surface elevation data. Scanning LiDAR consists of optics to allow the angle of the pulse exiting the sensor to change, and as the system scans back and forth, the specific angle of the pulse is used in combination with distance, GPS, and inertial measurements to determine the coordinates of the surface reflection point. The frequency of scanning determines the density of LiDAR return points with respect to the footprint of the laser pulse on the surface. Furthermore, high-angle pulses and topography can affect the quality of a given return.