Airborne laserscanning is a comparatively recent technology for capturing data of the topography of the earth, which became feasible through the availability of lasers with special attributes and the Global Positioning System (GPS).

In this method the distance between an aircraft and the earth’s surface is measured by determining the travelling time of a light pulse. In addition, an opto-mechanical device deflects the laser beam laterally into a slightly different direction for every measurement. In conjunction with the forward movement of the aircraft, a wide strip of terrain is scanned as a result.

To be able to generate a digital model of the topography, it is also necessary to accurately record the position and altitude of the aircraft, as well as the angle of every emitted measuring beam.
Due to the specific characteristics, this measuring method is distinguished from classical surveying techniques. The high density of measuring points and their uniform distribution are decisive for the high accuracy of the elevation model. The differentiation of the first and last light echo following a single measuring pulse permits identifying and determining further properties of the terrain.

The first echo is reflected by the ground surface, hence by treetops, high-voltage transmission lines or roof ridges, the last echo mostly by the land underneath. A Digital Surface Model (DSM) is generated by the selection of the first echo. The last echos are the starting point for generating the Digital Terrain Model (DTM). Given reflectors, such as house roofs or roads, both echos are identical.
A feature distinguishing the principle of laserscanning from terrestrial surveying, but also from photogrammetry, is that no selection of single prominent points takes place during measurement. The acquisition of areal data of extensive terrain is to the fore. Laserscanning is highly cost-effective in this connection because the processing sequence of the data can be very largely automated, from the acquisition in flight, through the evaluation, all the way to the end product of the elevation model.