What is Hillshade (Shaded Relief)?
Hillshade — also called shaded relief — is a display technique, not a measurement, that simulates how sunlight would fall on terrain to make its shape jump off the screen. A DEM shown as raw elevation colours can be hard to read; hillshade fixes that.
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.
Frequently asked questions
What is hillshade (shaded relief)?
Hillshade, also called shaded relief, is a display technique that simulates how sunlight would fall on terrain to make its shape easier to read than a DEM shown in raw elevation colors. It is a visualization technique, not a measurement.
How does hillshade actually work?
For each cell, hillshade computes how brightly a light source would illuminate it based on the local slope and aspect — slopes facing the light come out bright and slopes facing away are in shadow — producing a grey relief image that reads instantly as three-dimensional shape.
What do azimuth and altitude control in a hillshade?
Azimuth is the sun's compass direction and altitude is its height above the horizon. A low sun casts long, dramatic shadows that exaggerate relief, while a high sun looks flatter; a common convention lights the terrain from the north-west because it reads correctly to most viewers.
Is hillshade the same thing as sun-and-shadow analysis?
No. Hillshade is an interpretive display technique for making maps and DEMs readable, distinct from sun-and-shadow analysis, which actually measures sunlight hours over time including terrain shadow.
Sources
Emphasize terrain with hillshade in STREAM
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