Hillshading and Relief Representation

A hillshade is a grayscale raster in which each cell's brightness represents how directly a virtual light source would illuminate that point on the terrain surface, computed from the slope and aspect (compass-facing direction) derived at each cell of an input DEM. The calculation follows a simplified Lambertian reflectance model driven by two parameters describing the position of the simulated sun: azimuth, its compass bearing, conventionally defaulting to 315 degrees, or northwest; and altitude, its angle above the horizon, conventionally defaulting to 45 degrees. Slopes facing the light source render bright, slopes facing away render dark, and the resulting image gives even an untrained viewer an immediate, intuitive sense of ridges, valleys, and relief that a flat elevation raster alone cannot convey.