Hillshading

Hillshading, also known as shaded relief, is a cartographic technique used to convey three-dimensional terrain on a two-dimensional surface by simulating illumination of the landscape. It enhances the perception of form and terrain features such as ridges, valleys, and slopes by applying shading to a digital elevation model (DEM) according to a hypothetical light source.

The technique relies on calculating how surfaces would receive light from a specified direction, defined by azimuth (horizontal angle) and altitude (vertical angle). For each cell in the DEM, the local slope and aspect (the steepness and direction of the slope) are determined, and a hillshade value is computed from the angle between the surface normal and the light vector. The resulting brightness values are then mapped to grayscale or color intensities. Different software packages implement the calculation with slightly different formulas, but the principle remains the same.

Hillshade typically uses raster elevation data such as SRTM, ASTER, or LiDAR. Preprocessing may include filling sinks, filtering noise, and resampling to a common grid. The technique is often combined with color relief, contour lines, or transparency to convey multiple data layers.

Applications include topographic maps, landscape visualization for planning and recreation, geology and geomorphology, and hydrological analysis. Variants such as multi-directional hillshade or ambient occlusion can reduce the bias introduced by a single light source and produce more natural-looking relief.

Limitations include dependence on a chosen light source, potential to mislead about absolute elevations, and sensitivity to DEM resolution and shading parameters. It is, however, a widely used, simple, and effective method for illustrating terrain.