Creating hillshades with gdaldem

In the first part in this blog series we created a Digital Elevation Model (DEM) for Jotunheimen, a mountainous area in Norway. We’ll use this DEM to create hillshade or shaded relief, a popular cartographic technique to visualise terrain by modulating light and shadows on a map. 

GDAL, my favorite GIS Swiss Army Knife, allows you to create hillshade with gdaldem:

gdaldem hillshade -of PNG jotunheimen.tif jotunheimen_hillshade.png

This command will create this PNG image:

[ Download image ]

A virtual light source is placed above the DEM to calculate which areas are lightened up and which fall in the shadow. You can clearly see the numerous mountains and valleys. By default, the light source is placed in a top left position (azimuth = 315 degrees). Let’s try to move the light source to a bottom right position (azimuth = 135 degrees):

gdaldem hillshade -of PNG -az 135 jotunheimen.tif jotunheimen_hillshade_az135.png

[ Download image ]

You get the completely opposite effect, the valleys are percepted as mountain ridges and mountains appears as valleys (multistable perception illusion). I’ve computed a video with a 360 degree change of light source:


It’s easier to avoid relief inversion when the light source is changed gradually.

You can also change the altitude of light source (-alt) from 0 (ranking light) to 90 degrees (directly above the terrain). This video shows the effect:


Lastly, you can change the exaggeration (-z) used to pre-multiply the DEM elevations. This video shows a gradual change from zero to 3:1 terrain exaggeration, where mountains are three times higher:


Our hillshade is still a boring black and white image. In the next blog post we’ll create a color relief - hypsometric tints - for our terrain map.