A hillshade is a shaded-relief raster derived from a digital elevation model (DEM) by simulating how an illumination source lights the surface. Each cell's brightness is calculated from its slope and aspect relative to a defined sun position, producing the familiar gray, three-dimensional look of terrain.
Why it matters
Raw elevation values are hard to read, but a hillshade instantly reveals ridges, valleys, scarps, drainage patterns, and subtle landforms. In geology it helps trace lineaments, fault traces, and structural fabric, and it serves as a base layer beneath semi-transparent geological or thematic maps to give them topographic context.
A concrete example
The standard hillshade is controlled by two angles: azimuth (illumination direction, default 315°, i.e. from the northwest) and altitude (sun height above the horizon, default 45°). With GDAL you would run gdaldem hillshade input_dem.tif hillshade.tif -az 315 -alt 45 -z 1, where -z is the vertical exaggeration (z-factor). The z-factor must account for any mismatch between vertical and horizontal units — for example a DEM in meters but coordinates in decimal degrees needs a z-factor correction or the relief will look absurdly extreme.
Common pitfall
A single light direction can create relief inversion: ridges may read as valleys when illuminated from an unexpected angle, biasing interpretation. Using northwest lighting (the cartographic convention) reduces this, and a multidirectional hillshade blends several azimuths to suppress directional artifacts. A hillshade is a visualization, not an analysis surface — never measure slope or aspect from it.