Shaded relief terrain representation (also known as hillshading) is an alternative to isolines for representing surfaces. This technique capitalizes on they way the human brain uses information from shadows to construct a mental model of the terrain that is being depicted (see Figure 6.cg.11, below).
Although shaded relief images do not present precise terrain information (i.e., it would be difficult to look at the map and determine the elevation at a particular point, as one can easily do so with an isoline map), most map readers find it much easier to imagine what terrain looks like from a shaded relief image than from an isoline map. Today, cartographers often combine both types of terrain representations to take advantage of the strengths of both types of representations (see Figure 6.cg.12, below).
Although early shaded relief maps were created by manually drawing or airbrushing shadows, today it is quite easy to use digital techniques to create high quality maps. Digital techniques typically begin with a digital elevation model (i.e., a raster file of elevation at regularly spaced points across a surface). Most methods require the cartographer to specify where the light source (i.e., the sun) is located, the elevation of the sun in the sky, and optionally, the level of vertical exaggeration. These factors are used in combination with the elevation values stored in the DEM to calculate how much light would be reflected at each pixel. Below, we explore the visual effects of changing some of these parameters.
In the example in the upper left in Figure 6.cg.13, above, we have set the location of the sun (i.e., the azimuth or angle of illumination) to 315° (the northwest corner), the standard location that cartographers use for creating shaded relief. Also in Figure 6.cg.13, in the upper right, you can see a shaded relief map of the same location, but that was created using a southeast azimuth (135°). Notice that features that appeared as ridges in the northwest-illuminated map (the map on the left) now appear as valleys, and the features that appeared as valleys now appear as ridges or slopes. Generally, if you use a light position that is in front of the viewer (i.e., to the north if we imagine that the viewer is at the center of the map), terrain will appear with hills and valleys in their proper locations, while if you use a light position that is behind the viewer (i.e., as in the southeast-illuminated image), terrain features may appear to be reversed. However, this result is highly dependent upon the position and orientation of the terrain features that occur in the area for which you are creating shaded relief. For example, if you had a ridge that runs from the northwest to the southeast, northwest illumination would not depict this feature very well, as both sides of the ridge would be illuminated equally. You can experiment with changing the angle of illumination and its effect on the visual appearance of shaded relief at http://www.reliefshading.com/design/lightdirection.html.
The sun altitude that you specify also plays an important role in determining the appearance of shaded relief. You can understand the basic effect of sun altitude on the shape and size of shadows by thinking of how your own shadow's appearance changes throughout the day: at noon, your shadow is quite short and compact, while as the afternoon progresses, the length of your shadow will slowly increase as the sun sets (i.e., as the sun's altitude decreases). This is also reflected in Figure 6.cg.14, above: in the first of the three images, at the top, we set the sun altitude to 90° (directly overhead), and you can see that there is generally little shadow in the image and it is difficult to discern where terrain features such as ridges are located; in the second image, the middle map, the sun altitude was set to 45°, providing enough shadow to easily detect major terrain features; in the third image, at the bottom, the sun altitude was set to 20°, and you can see that there is a large increase in the overall amount of shadow present in the image, even to the point that many valley bottoms are almost completely obscured by shadow. Although a low sun altitude may be inappropriate for some areas (especially those with significant elevation changes), using a low sun altitude may be quite useful in bringing out more subtle terrain features, particularly in areas of lower relief.
Cartographers often use vertical exaggeration in an attempt to match the map reader's subjective impression of terrain differences; because people are generally quite small in comparison with terrain, the small apparent differences in shaded relief (that reflect reality) may often appear to be smaller than the features are in the map reader's mental image or memory of the terrain (Robinson et al. 1994). In the images in Figure 6.cg.15, above, you can see the main visual difference that changing the vertical exaggeration causes: the lengthening or deepening of shadows. At the left is an unexaggerated scene; at the right is an image that exaggerates the terrain by a factor of two. Although terrain exaggeration is not really necessary for this location (because of its substantial relief), it can be very useful for depicting subtler features in areas with smaller elevation differences.
A final technique that cartographers often use for enhancing shaded relief maps is the use of a hypsometric tint (see Figure 6.cg.16, above). A hypsometric tint is a color ramp that is overlaid on a shaded relief image (or used to fill isolines as in the example shown in Isolines) to add information about elevation to a map. Although it may still be difficult for the map reader to determine an exact figure for the elevation at a particular point from the shaded relief, the hypsometric tint does help the map reader develop a rough understanding of the height differences portrayed in the map.
Basic hillshading (such as the examples shown above) can be quite easily accomplished within most GIS packages today, but fine refinements (especially those that are more important for process printed maps), such as varying the resolution within a DEM (to accommodate the need for displaying varying levels of terrain variability), resolution bumping (merging of low and high-resolution data), or using illumination from multiple light sources to ensure that all important terrain features are visible are usually completed in graphics packages such as Adobe Photoshop.
If you are interested in investigating this subject further, I recommend the following:
- Patterson, T. 2004. "Shaded relief: Ideas and techniques about relief presentation on maps.".
- Jenny, B. and S. Raeber. 2004. http://www.reliefshading.com.