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Thursday, 21 June 2012

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:


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


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.

8 comments:

vncbmerter said...

Hi, nice post.
Here is a very handy tool to create this kind of images. It is based on gdaldem and provides a preview of the result : http://spaceyes3d.com/plugin/index.html
The tool is installed along the free 3D viewer (allowing to drape an image over a DEM).
It is also possible to apply a colormap to the DEM (featuring an interactive editor).
Windows only for now.
Regards,
Fred.

Mike said...

Beginners question: in what program did you run this code? Thanks!

gdaldem hillshade -of PNG jotunheimen.tif jotunheimen_hillshade.png

Bjørn Sandvik said...

Hi Mike,

You need to install GDAL: http://www.gdal.org/

If you're on Windows you can install OSGeo4W:
http://trac.osgeo.org/osgeo4w/
(select Express Desktop Install)

Mike said...

Thanks a lot. Another beginner's question: How do you tell OSGeo4W where to look for your files? I've tried:

~Users\Public\mikedata$

Bjørn Sandvik said...

You run OSGeo4W Shell - and then you can run the GDAL commands anywhere. Go to the folder with your data files.

Introduction to the Windows Command Prompt

Mike said...

Thanks for the link. I am quite a beginner.

DR said...

Hi, Bjørn

What is software did you use for making series of images with different key values of gdaldem?

and azimuth = 135 is right bottom place, not left bottom.

Bjørn Sandvik said...

Hi DR,

I used a batch script:

FOR /L %%i IN (1,1,360) DO (
gdaldem hillshade jotunheimen.tif azimuth/%%i.tif -az %%i
)

+ Windows Live Movie Maker

I've fixed the typo. Thanks!