Colour mapping can give a general impression of the topography, but it doesn’t look plastic, natural, real: in the real world, landscapes are usually not coloured according to elevation. When looking at a sunlit landscape from above (i.e., as in looking at a DEM image), we realise that much of the visual impression is created by illumination. This is the of the commonly used DEM visualisation technique known as shaded relief (also hillshading or cosine shading).
Imagine a surface illuminated by a single light source. Or try it in a darkened room with a flashlight. If the light shines vertically onto the surface, the surface will be bright. If the light illuminates the surface at a shallow angle, the brightness of the surface will diminish because the same amount of light is distributed over a larger surface are. Mathematically, the amount of light received by a given surface area is proportional to the sine of the angle between the beam of light and the surface. Expressing the same fact by referring to an imagined line that stands vertically on the surface (the surface normal), the amount of light received by a given surface area is proportional to the cosine of the angle between the beam of light and the surface normal.
In our everyday experience, objects are usually illuminated from above. Most people are right-handed, so when handling objects, we tend to hold them in a way that they are illuminated from the left to avoid shadowing. Over years and decades of experience, this becomes so much engrained in our brains that illuminations differing from the above left scheme can cause optical illusions. In a shaded relief image with simulated illumination from “below” (i.e., south), mountains and valleys may be perceived as inverted.
Because shaded relief images can convey a lot of topographic detail, traces of human activites may become visible. In the examples below, forestry roads wind around the mountains, and numerous roughly circular features dot the slopes. These features are charcoal burning platforms which document an early modern forestry use quite different from today’s. Note that the visibility of charcoal burning platforms on differnet slopes of the mountains depends on the simulated illumination direction.
In the case of anthropogenic features cut into relatively steep slopes (such as forestry roads and charcoal burning platforms), vertical illumination works quite well to make these features visible. Applying a vertical exaggeration to the DEM enhances visibility even more.
Vertical illumination and/or vertical exaggeration don’t always help to improve relief feature visibility. In mountainous areas such as the Black Forest, they can be very useful. In flat areas such as the Upper Rhine Valley, low angle illumination directions (which would in turn not be very useful in the mountains) are much better suited to show topographic detail.
A particular case are linear relief features. Brightness changes in shaded relief images are caused by changes in surface slope and direction relative to the simulated illumination direction. Therefore, linear features aligned perpendicular to the illumination azimuth are characterised by much strongewr contrast than linear features aligned parallel to the illumination direction.
Imhof, E., 2007. Cartographic relief representation. English language edition edited by H.J. Steward. ESRI Press, Redlands.