Aspect is the compass direction that a slope faces. Derived from a digital elevation model (DEM), it is the horizontal orientation of the steepest downhill direction at each cell, expressed in degrees clockwise from north (0° = north, 90° = east, 180° = south, 270° = west).
Why it matters
Aspect controls how much solar radiation a slope receives, which in turn influences soil moisture, vegetation, snowmelt timing, weathering, and microclimate. In the Northern Hemisphere, south-facing slopes are warmer and drier while north-facing slopes are cooler and wetter. Geologists and terrain analysts use aspect to interpret differential erosion, map slope-related hazards, plan solar or infrastructure siting, and segment landscapes into homogeneous units.
A concrete example
Aspect is computed from the same 3x3 neighborhood gradients used for slope, applied to a DEM such as a 30 m Copernicus or SRTM grid. In QGIS or with gdaldem aspect, output is degrees clockwise from north. Flat cells (zero gradient) have undefined aspect and are conventionally assigned a value of -1 so they can be masked. Because aspect is circular, it is often reclassified into cardinal/intercardinal bins (N, NE, E, …) for mapping rather than averaged numerically.
Common pitfall
Aspect is circular data, so you cannot take a plain arithmetic mean: averaging 350° and 10° naively gives 180° (south) when the true mean is 0° (north). Use circular statistics, or split aspect into sine and cosine components, when summarizing it. Also remember that aspect is meaningless on flat ground.