A set of (X, Y, Z) coordinates is a model of the terrain surface
When mounted on an aircraft, a laser rangefinder measures at very short time intervals the distance to the terrain. Combining a laser rangefinder with sensors that can measure the position and attitude measuring by three sensors of the aircraft (GPS & IMU) makes it possible to create a model of the terrain surface. We can define the coordinate system in such a way that Z refers to elevation. The digital surface model (DSM) thus becomes a digital elevation model (DEM), i.e. we model the surface of interest by providing its elevation at many points, each with position coordinates (X, Y ). Since the “raw DEM” gives us elevation of the surface the sensor “sees” (Figure 4.22). Post-processing is required to obtain a digital terrain model (DTM) from the DSM.
Model of the terain surface