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Shaded Relief Maps
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Shaded Relief Maps

Shaded relief maps are raster maps based on grid files. These maps use colors to indicate the local orientation of the surface relative to a user-defined light source direction. For example, Surfer determines the orientation of each grid cell and calculates reflectance of a point light source on the grid surface. The light source can be thought of as the sun shining on a topographic surface. Portions of the surface that face away from the light source reflect less light toward the viewer, and thus appear darker.

The colors on a shaded relief map are based on the reflectance from the grid surface. Reflectance values range from zero to one. A reflectance value of zero means that no light is reflected toward the viewer. A reflectance value of one means that all incident light is reflected toward the viewer.

Because colors are assigned to entire grid cells, sparse grids (grids with relatively few rows and columns) are generally poor candidates for shaded relief maps. Shaded relief maps based on grids with too few cells look blocky or fuzzy.

With a good software package such as Surfer you can assign colors to represent the various reflectance values. Colors between the assigned values should be automatically blended to make a smooth gradation. Surfer, by way of example allows two different methods to make a smooth gradation; "central difference" and midpoint difference". The Central Difference method averages the slope and orientation of the surface across three adjacent grid nodes. This results in a smoother shaded relief surface but because a node on either side is required, the edge cells are blanked. The Midpoint Difference method computes the gradient at the center of each grid cell. This method provides less smoothing, but does not blank the grid cells at the edge of the map.

When it comes to shaded relief maps, the shading method used can be critical and experimentation with your data is recommended.

A "simple" shading method is normally fastest, but often produces a rather crude image. Other, more advanced methods such as using Peucker’s Approximation (a piecewise linear approximation) give somewhat better results, but redrawing the map can take slightly longer.

Other shading methods include Lambertian Reflection (assumption of an ideal surface that reflects all the light that strikes it; the surface appears equally bright from all viewing directions) and the Lommel-Seeliger Law method which is based on an analysis of light scattering from a surface. The Lommel-Seeliger Law method makes a compromise between an ideal diffuser and a real surface. With some surfaces, this may actually give better results than the Lambertian Reflection method. Again, experimentation with your data is recommended.