Hill shading and the reflectance map

Shaded overlays for maps give the user an immediate appreciation for the surface topography since they appeal to an important visual depth cue. A brief review of the history of manual methods is followed by a discussion of a number of methods that have been proposed for the automatic generation of shaded overlays. These techniques are compared using the reflectance map as a common representation for the dependence of tone or gray level on the orientation of surface elements.