ANINTRODUCTIONTOGIS


Terrain analysis with Digital Elevation Model


A Digital Elevation Model (DEM) is a raster in which each pixel gives the altitude at a given point. A DEM may be used in Geographic Information Systems (GIS) to produce various maps, such as relief maps.

An example of DEM to perform this tutorial is available here.

1. Create a 3D relief map from DEM

Start by loading the DEM into your project:

  • Layer > Add Layer > Add Raster Layer

Practise: Choose an appropriate color scale to display the elevation for the area of interest

Several layers can be created from this DEM. To do this, open the raster terrain analysis menu:

  • Processing > Toolbox
  • Search for “Raster terrain analysis”

We may now create a Hillshade, which is a raster that maps the terrain using light and shadow to create a 3D-looking image.


Figure 1 Hillshade creation


The algorithm allows you to specify the position of the light source: Azimuth has values from 0 (North) through 90 (East), 180 (South) and 270 (West), while the Vertical angle sets how high the light source is (0 to 90 degrees).

We may add some transparency to the hillshade:

  • Right click on the layer > Properties
  • On the Transparency tab, set Global opacity to 40%

The hillshade created may be used to improve the rendering of other projects.

Practise: Open the vector layer of the municipalities of used in the first tutorial. Keep only the borders of the municipalities, with a transparent filling. Below, add the hillshade layer.


2. Create a slope map

In the Raster Terrain Analysis toolbox, select Slope to make a slope map from the DEM raster.

Practise: Identify the area with the steepest slope.


3. Identify north-facing slopes from aspect map

In the same way as above, create an Aspect map from the DEM raster.

We will now use this aspect map to identify north-facing slopes.

In aspect map, North is at 0 degrees, so for the terrain to face north, its aspect needs to be greater than 270 degrees or less than 90 degrees.

We will select the corresponding pixel with the Raster Calculator:

  • Raster > Raster Calculator


Figure 2 Select north-facing slopes


In the resulting raster, north-facing pixels are identified with the 1 value.