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,
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
you can assign colors to represent the various
reflectance values. Colors between the assigned values should be automatically blended
to make a smooth gradation. , 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 Peuckers 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