LIDAR (Light Detection and Ranging or Laser Imaging Detection and Ranging) is a new technology where a laser beam from an aircraft platform scans the ground perpendicular to the line of flight. The laser returns or reflections from the ground and foliage are processed to determine the ground elevations. LiDAR is cost-effective and a more accurate process for determining elevations than the photogrammetric compilation. LiDAR has the ability to penetrate tree cover, thereby providing the most accurate ground definition possible.
We use a Leica ALS50-II laser system consisting of the source, scanning assembly, and timing electronics; a positioning and orientation sub-system comprising the differential GPS and inertial measuring units; a data storage unit, and processing software. This is the most advanced technology currently available in the world.
The Leica ALS50-II system offers technical and operational advantages not available from any other system. It has been designed and operates in a manner that optimizes all critical considerations of a LiDAR system and survey as follows:
- The ALS50-II has, by a factor five times, the highest output power of any other system on the market. This provides two critical operational advantages; i.e. this allows the system to be flown at a high altitude and still maintain very high pulse return data. The ALS50-II is flown as much as four times higher than any other system and this results in wide swath width and ground coverage thereby significantly reducing cost. Secondly, the higher power output provides superior foliage penetration, particularly in dense foliage.
- The ALS50-II system records all raw data from the inertial and GPS units allowing for detailed post processing of the aircraft position and sensor orientation. A number of commercial LiDAR systems do not record raw data resulting in a significant degradation in position and orientation accuracies.
- The ALS50-II system scans back and forth across the flight path ensuring that all laser pulses are transmitted out of the aircraft to the ground. A large number of commercial LiDAR systems use a rotating mirror system rendering as much as 60% of the LiDAR pulses useless (since they are not directed at the ground).
- The ALS50-II is the first LiDAR system to offer multiple pulse in the air technology. This significantly boosts the pulse density and provides a better surface representation at faster aircraft speeds.
The ALS50-II system offers performance far beyond that achievable by any other airborne LiDAR system operating today. The system develops a sinusoidal scan pattern on the ground with a variable field of view from 10° to 75° and operates at altitudes from 600m to 6,000m, with a swath width of 150m to 10,000m. The point density is a function of the aircraft speed, field of view, and altitude. Typically, the aircraft operates at 2,500m above ground with a scan swath of 1,100m.
The GPS/INS airborne position and orientation solution is used to determine the scanner position and sensor orientation to compute the position of the laser spot on the ground. The appropriate transformations are used to derive the final data product in the user-specified horizontal and vertical datum. Obstructions and vegetation are removed during the post-processing phase (if required) and the data are closely examined for anomalies. The resulting LiDAR image is a DEM.
TerraScan is used for the initial removal of vegetation and man-made features. TerraScan produces a map of the topography geo-referenced to a UTM zone on NAD 83 (horizontal) and NAVD 88 (vertical) datum. TerraScan can perform a statistical comparison between the control positional information and the LiDAR data. Specifically, TerraScan searches for LiDAR positions offset from control positions by a defined distance and returns the X, Y, chord, Z offset, and a statistical summary.