The Aspect (Terrain analysis) tool uses a DEM to calculate the direction in which slopes are facing (see section for more information).To calculate the aspect based on a DEM:

  1. Go to Raster.
  2. Select Analysis.
  3. Choose DEM (Terrain models).
  4. Choose the mode Aspect (Fig. 103) (the function Raster > Terrain analysis > Aspect is just another option to achieve the same result).
  5. Select the DEM as the input raster.
  6. Browse for a destination path to save the output raster as ASTER30_aspect_CH1903.tif.
  7. Some optional settings can be activated or deactivated in the checkboxes, to make flat areas (that do not have a valid aspect value by default) zero values, for instance. 
  8. Click OK.
  9. Adapt the raster symbolization if necessary (Layer Properties > Symbol).

Fig. 103. Calculating aspect with QGIS.