AIRBORNE SCANNING LASER ALTIMETRY

摘要

Laser altimeters use the principle of lidar (Light Detection and Ranging) to calculate the distance between an aircraft and the ground. The round trip time of flight of laser pulses emitted by an onboard laser is precisely measured and converted to a range based on the speed of light. First used in the 1960s as navigation instruments, lidars have recently become a key component of scanning laser altimetry (SLA), a technique that is now revolutionizing topographic mapping. By combining the data generated by a high frequency pulsed laser, a Global Positioning System (GPS) receiver, and an accurate inertial measurement unit (IMU), the precise three-dimensional position of the laser echoes on the ground can be calculated. Airborne SLA systems currently allow the acquisition of up to 100 000 xyz data points per second with an accuracy of approximately 15 cm. This signifies for example that a 1 km2 surface can be precisely surveyed with a density of two points per square meter in no more than 20 seconds. The highly collimated laser pulses have the capacity to penetrate vegetation canopies. If a significant amount of pulse energy reaches the ground level, the last return detected will correspond to the elevation of the ground. SLA can thus reliably map terrestrial elevations under dense forests. By improving on the traditional methods of photogrammetric reconstruction of topography from stereo aerial images, SLA is now setting higher resolution and accuracy standards for the three-dimensional mapping of the earth's surface. The digital elevation models (DEM) created using SLA data are used to improve flood risk mapping, establish the mass balance of glaciers, map vegetation biomass, or reveal hidden geological structures. Canada is recognized as the world leader in SLA technology thanks to the Toronto firm Optech Inc., the largest manufacturer of SLA systems. This paper will describe the airborne scanning laser altimetry technology and principles, explain how the data is acquired and processed, and present some of its applications.