Taking the Best of Both Worlds for a Better Hillshade

A popular cartographic element is the venerable hillshade. With a good hillshade and a transparent overlay, you can add a lot of topographic context to your map without being overly distracting. Most of us have probably used a  bare earth hillshade before. These are great if you just want to look at the underlying terrain, or if you are covering a wide area.

For close-in work, creating a hillshade from DEM that includes the vegetation canopy can make things much more interesting. However, if you look closely at the canopy hillshade, you’ll notice that many of the terrain features no longer stand out, even where there is little to no vegetation. This is because individual trees and shrubs create more steep areas, which proceed to hog the dark end of the stretched histogram. So what’s a cartographer to do?

With a little mathemagic, you can have your cake and eat it, too. My workaround is to average the values of the two hillshades, which re-introduces the steep areas from the bare earth DEM. Any averaging technique will work, but I prefer the geometric mean (nth root of the product of n values), as it does a good job of normalizing the values. Just open up the raster calculator in your GIS of choice, and plug your hillshades in to the formula for your mean of choice. Here is the result of the geometric mean of the hillshades above:

hybridhs

Look how nicely the vegetation and terrain come through in these maps of canopy height and slope. So, next time you want to wow someone with a map that’s close in, try a hybrid hillshade. It’s easy to do, and will really set your work apart.

canopyheight
Canopy height with hybrid hillshade.
slope
Canopy height with hybrid hillshade.
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Author: jcguarneri

Mapmaker, bicycle advocate, fisherman, naturalist. Follow me on Twitter @jayguarneri.

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