Hillshade, also called shaded relief, is a way of drawing terrain by working out how light from one direction would light and shade each slope. It makes hills, ridges and trenches look three-dimensional on a flat screen.
Key takeaways
- Hillshade is calculated from a grid of heights: slopes facing the imagined light are bright and slopes facing away are dark.
- This map draws shaded relief from the GEBCO_2026 grid and colours it by depth.
- Hillshade shows the shape of the terrain but not its height, which is why colour is added.
- Because the light is imagined, a ridge can look like a trench if you imagine the light coming from the wrong side.
How hillshade works
Hillshade starts with a grid of heights, a digital elevation model. For every cell, a computer works out the slope and which way it faces. Then it picks an imaginary light source, usually low in the sky towards the upper left, and sets the brightness of the cell to match. A slope facing the light is bright. A slope facing away is dark. Flat ground sits in the middle.
The effect is that a flat picture seems to have relief. Your eye reads the bright and dark sides the way it reads a real hillside in low sun. Artists drew hachures and shadows by hand to get the same result, but a computer can do it for every cell of a grid in an instant.
The ocean floor is a good use for it, because nobody can photograph the bottom of the sea from above. Bathymetric maps normally show the shape of the seabed with colour or contour lines [2]. Shading gives you the texture that colour alone hides: the jagged crest of a ridge, the smooth floor of a plain, the sharp edge of a trench.
Hillshade only shows shape. It does not say how high or deep a place is. That is why this map combines it with colour. The shaded relief is drawn from the GEBCO_2026 grid [1], and the colour tells you the depth. The read the map section of the homepage explains what each colour means.
Two limits are worth knowing. A grid cell averages the seabed over a whole cell, so the shading is smooth at that scale and cannot show anything smaller. And in the open ocean, where the grid is partly estimated from satellites, shading shows the estimated shape, not a surveyed one.
An example from our data
The Mid-Atlantic Ridge is a good place to see hillshade at work. Its long crest runs down the middle of the Atlantic Ocean, and the shading brings out its rough texture and the many cracks that cut across it. The deepest cell the grid finds near the ridge reads 1,477 m.
Compare that with the Mariana Trench, where the shading picks out a steep, narrow edge, and the deepest cell reads 10,931 m. Both sit under an ocean that looks smooth and grey in a photograph from space, but the shaded relief makes their shape obvious.
The thing people get wrong
People often take a hillshaded map for a picture of the real seabed. It is a picture of a calculation. The light is imagined, and the same terrain shaded from a different side looks different. If you light a ridge from the wrong direction, your brain can flip it into a trench. That is why most maps put the light at the upper left.
Brightness also has nothing to do with depth. A dark patch is not necessarily deep. It is a slope facing away from the imagined light.
Hillshade vs colour by depth
| Hillshade | Colour by depth | |
|---|---|---|
| What it shows | The shape and texture of slopes | How deep each place is |
| How it is made | From slope and an imagined light | From the height value of each cell |
| Why combine them | Shape without depth is misleading | Depth without shape looks flat |
Frequently asked questions
Is hillshade a photograph?
No. It is calculated by a computer from a grid of heights. There is no camera, and the light is imagined.
Why does the light usually come from the top left?
Because people read relief most naturally when the light seems to come from the upper left. Map makers copy that habit so slopes look raised rather than sunken.
Does hillshade show the real colour of the seabed?
No. The seabed is dark and the colours on this map are added to show depth.
Sources
- GEBCO_2026 Grid, GEBCO.
- What is bathymetry?, NOAA Ocean Service.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).