Airborne LiDAR Mapping
Airborne lesar mapping. (LIDAR - Light Detection And Ranging) is a fast and reliable method of obtaining 3-dimensional data for the creation of a Digital Terrain Model (DTM).
Capable of producing a digital terrain model to an accuracy of +/- 15-cm, the system is extremely useful for applications where a relatively high degree of accuracy is necessary. In addition, a laser digital elevation terrain model can be produced in a much shorter time frame than a similar product using conventional photogrammetric techniques. The ALTM system is comprised of a high frequency optical laser coupled with GPS and an Inertial Navigation System (INS). A 3-dimensional GPS solution (X, Y, Z) is used to position the laser scanner at each second or half second, while the INS data are used to determine the systems' orientation. The GPS solution is computed from differential kinematic processing, using data collected simultaneously at the aircraft, and at base stations on or near the project site.
From the airborne platform (fixed wing or helicopter), the laser emits pulses at frequencies of up to 5000 Hertz. These pulses are reflected off vegetation or man-made structures at different time intervals, so the varied distances between the emission and reception can be calculated. With such high pulse emission rates, the laser can obtain as many as 300,000 3-D points per minute. Depending on the flying height, the aircraft speed, and the frequency and width of the laser scan, the ground point density can be as close as 2 feet. When care is taken to ensure quality lidar data collection and high percentages of laser returns, an accuracy of +/- 6 inches is commonplace. Depending on the accuracy requirements of the digital terrain modeling, the aircraft can be flown up to an altitude of 3,200 feet, producing a 2,100 feet wide laser swath. With a higher flying height, however, there will be a decrease in the density of the points, which in turn will affect the accuracy of the resultant DTM. Typical flying heights to achieve 1 foot contours are around 1,600 feet above ground level (AGL).