Technology of ALS

ALS, also referred to as LiDAR (Light Detection and Ranging), is an active remote sensing technique. The laser scanner is usually mounted below a fixed wing aeroplane or helicopter, where it emits short infrared pulses towards the earth’s surface into different directions across the flight path (typically 30,000 to 100,000 pulses per second). Each pulse will result in one or more echoes reflected from various objects along its path (vegetation, buildings, cars, ground surface etc.). While in theory a good deal of first echoes return from the top of trees and buildings (off-terrain points), the last one should be returning from the ground surface (terrain points). The location of each reflecting object is calculated using the angle of the emitted laser beam, the distances to the reflecting object (measured by the time delay between emission and each received echo), and the position of the scanner (typically determined using differential global positioning system (dGPS) and an inertial measurement unit (IMU)).

Because of the high density of measured points and their more or less even spread, the digital terrain models are very precise. The absolute accuracy usually lies within a range of +/- 20 cm in plain and +/- 15 cm in height. The vertical relative accuracy, i.e. the minimal vertical extension of objects lies between 5 and 10 cm.