What is Hillshade (Shaded Relief)?
A DEM shown as raw elevation colours can be hard to read. Hillshade — also called shaded relief — makes the terrain's shape jump off the screen by simulating how sunlight would fall on it. It is a display technique, not a measurement.
Simulated sunlight
Hillshade computes, for each cell, how brightly a light source would illuminate it given the local slope and aspect. Slopes facing the light are bright; slopes facing away are in shadow.
The result is a grey relief image that our eyes read instantly as three-dimensional shape.
Azimuth and altitude
Two settings control the effect: the sun's azimuth (its compass direction) and its altitude (its height above the horizon). A low sun casts long, dramatic shadows that exaggerate relief; a high sun is flatter.
A common convention lights the terrain from the north-west, because it reads correctly to most viewers.
Display, not analysis
Hillshade is about interpretation — it makes maps and DEMs readable. It is distinct from sun-and-shadow analysis, which measures actual sunlight hours over time including terrain shadow.
Emphasize terrain with hillshade in STREAM
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