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The light direction is very important for the design of a shaded relief. Normally the cartographer lets the light "shine" on the terrain from the upper-left. Less popular is illumination from the south, as the relief shading tends to not "look right". In extreme cases, relief inversion occurs where mountains appear as valleys and vice versa.
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Northwestern illumination.
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Southeastern illumination.
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If executed carefully, a shaded relief with southern illumination may depict the terrain with vivid and correct form. However, maps with southern illumination are generally more difficult to read. For a comparison, see the two maps of the Canton of Grisons below. Also see "Rigi" by Fridolin Becker for an example of a successful map with a southern illumination.
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Northwestern illumination. "Graubünden" (section), 1:250 000, 1945.
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Southern illumination, "Carte de voyage des grisons" (section), 1:250 000, 1932.
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Adjustment of the Light Direction
On a typical shaded relief, the optimal main light direction strikes mountain ridges and valleys at an angle of approximately 45°. Emphasizing and clarifying important topographic features, such as ridges, valleys and watersheds, requires adjusting the direction of illumination to the characteristics of a given terrain.
The top-left image below shows a stylized terrain under strict northwestern illumination. On the top right image, adjusting the direction of illumination results in light striking the ridge from the top in the western part, and from the left in the eastern part. This adjustment avoids "dead" mountain ridges in transition zones, where the illuminated and the shaded sides of a mountain share the same shade of gray.
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Stylized terrain without local adaptation of the light source.
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Terrain with local adaptation of the light source.
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Analytical shading with illumination strictly from northwest (DEM © swisstopo).
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Interactive Java Applet (DEM © swisstopo).
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| The lower left image shows a digital elevation model rendered with strict northwestern illumination.
Move the little sun on the lower right image to change the light direction. Observe how certain parts of the terrain are well depicted (and others are not), depending on the chosen light direction.
You may further explore the influence of the light direction with another interactive Java Applet and a digital elevation model of the Grand Canyon, USA.
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