Why Does The Sunlight Come From The North In Shaded Relief Maps?

If you've got a sharp eye and a knack for details, you might have noticed the sun coming from an unrealistic angle in most shaded relief maps: the north. But in the northern hemisphere, sunlight mostly comes from the southern half of the sky, so what gives?

A map of Going-to-the-Sun Road in Glacier NP, by Tom Patterson. The sunlight comes from the upper left in this lovely map.

It's Because of a Quirk of Human Perception

When you take a look at this image, is the lightest gray side on the top or the bottom of the cubes?

At first glance, is the lightest side on top or bottom of the cubes?

Most folks see the lightest gray side to be the top of the cube. This is because when interpreting shadows the human brain tends to assume light comes from above.

For this reason, shaded relief maps typically have the sunlight coming from the top of the image so that the terrain is interpreted correctly.

Lit From Below, Terrain Gets Inverted

Because our brains are used to light coming from above, when terrain is lit from below our brain sees things as inverted.

Check out the below example. Is the location marked with an "X" on a ridge or in a valley?

Is the "X" on a ridge or in a valley?

Most people will see "X" as a ridge, but it's actually in a valley! If you follow that valley to the right you can see it run into a larger river, which appears to be sitting on top of another ridge!

Here's how that same area looks with the hillshade coming from above the image.

Only the sunlight angle has changed between this image and the last one.

Now this looks better! The rivers aren't running along ridges anymore, but at the bottom of valleys. All we did was change where we positioned the sun when we generated the hillshade.

Perceptual Accuracy, Instead of Physical Accuracy

It's true, in the northern hemisphere the sun mostly comes from the southern half of the sky, so the sun placement in most shaded-relief maps in the northern hemisphere isn't realistic. But we make maps for humans and this is how the human brain works!

The less work your brain needs to do to look at a map, take it all in, and see the terrain for what it is, the better.

If you've read this far you should check out our available maps. I'm really proud of how our images look on our website but I promise, they are even more incredible on the wall!