3.2.4. Digital Elevation Model (DEM)

A DEM (Fig. 7) is a raster dataset that represents the altitude of the terrain. Depending on the resolution of the image, each pixel represents the average altitude of the terrain covered by that pixel. For example, if the image resolution is 30 m, the altitude of the terrain on the ground that is covered by that pixel within that 30 m square is averaged and the value is assigned to that pixel.

With a DEM, numerous calculations can be performed including the calculation of slope (in degrees or percent) of the terrain, the direction each slope is facing (aspect), and where shadows exist when the sun is hitting the terrain from a certain angle (hillshade). A DEM is also the base dataset for calculations of solar radiation, hydrographic, and various modelling calculations.

Fig. 7. A sample Digital Elevation Model (DEM) (30 m resolution) covering the study area of Fribourg, Switzerland. This map was a free download from ASTER GDEM which was projected to the Swiss projected coordinate system (CH1903 LV03).