In laser mapping using an ALTM (Airborne Laser Terrain Mapper) system, the laser beam is deflected by an oscillating mirror (scanner) across the flight line, so that a swath of terrain along the flight path is sampled by a zig-zag line. The distance to the ground surface is determined by runtime measurement. The external orientation, i.e. the position and orientation of the sensor in space, is determined by means of GPS (Global Positioning System) and INS (Inertial Navigation System).

This information, combined with the mirror angle measurement, allows to calculate for each reflected laser pulse its position, i.e. its easting, northing and altitude on the surface.