Calculating slope and aspect from DEMs Elevation metres

Calculating slope and aspect from DEMs Elevation (metres) Slope and aspect can be automatically calculated from a DEM. To illustrate how this works, let us ‘zoom in’ just 9 grid cells within a DEM.

Calculating slope and aspect from DEMs Elevation (metres) W S N E Consider the middle (green) square in this example. It is surrounded by higher ground to the north and lower ground to the south. The ground to the east and west has the same elevation.

Calculating slope and aspect from DEMs Using the elevation values for all the grid cells, we can fit a plane to this surface. This plane is known as a hypsometric curve. W N From this plane, we can α E S Hypsometric curve calculate the angle of slope (α) and the aspect (direction of slope) for the middle grid cell. In this case, the aspect is south-facing, since there is higher ground to the north and lower ground to the south.

Calculating slope and aspect from DEMs Centre grid cell Of course, in reality, it is unlikely that our elevation values will be as regular as the previous example suggested. In this example, the 8 grid cells surrounding the centre cell all have different elevation values.

Calculating slope and aspect from DEMs Nonetheless, it is still possible to calculate a hyposometric curve from their elevation values, and so estimate slope and aspect again. Hypsometric curve

Calculating slope and aspect from DEMs We can repeat this calculation for each grid cell and its 8 neighbours. In this way, we can map the slope and aspect for the entire study area. slope (degrees)