Recently I stumbled across a couple of very interesting questions over on the GIS Stack Exchange. One of them was about the use of appropriate colour palettes for displaying DEMs, and the other was about colour palettes for hillshade images. This last one in particular got me thinking a lot about effective cartographic display of terrain and introduced me to the work of the famous Swiss cartographer Eduard Imhof. Why is it that every hillshade image that I have ever created is displayed in a greyscale palette?
I’ve always thought that hillshading is a very effective way of visualizing topographic information, but greyscale hillshade images by themselves are so boring! Of course the answer is that I don’t normally just display a hillshade image. Instead, the hillshade image is displayed transparently over top a DEM, with colour used to relay elevation information and tone used for illumination. This type of composite relief model is very effective for visualizing terrain. But what about using other colours for illumination other than simply greyscale hillshading, like Imhof did so long ago? To do this effectively in Whitebox, you may want to create a custom palette using the Palette Manager (under the Tools menu), one of the most under-used tools in Whitebox:
So here is a hillshade image displayed using the custom palette above [simply blending RGB(0, 50, 100) to RGB(255, 240, 170)]:
I think that’s beautiful. It’s like bathing the landscape in warm, bright sunlight. Of course, we can actually take this a step further. My father is an artist and retired art teacher. I remember when I was growing up, he taught me that the atmosphere is actually coloured when you have enough air thickness; it’s a little blue. That’s why in this painting of his, one of my favourites, you can see that the hills in the distance become slightly bluer:
I now know that this phenomenon is called Rayleigh scattering (the reason that the sky is blue), and koodos to my Dad for inherently understanding this phenomenon. So since the height of the air column and density of the atmosphere are greater in the valley bottoms, there should be a ‘blue tinge’ in the valleys. We can create this ‘blue tinge’ by transparently overlaying the DEM, displayed in a light-blue-to-white palette (again created using the Palette Manager), and get this:
You can play around with the palettes and the levels of transparency (and display minimum and maximum values) until you get things just the way that you want. It’s almost artistic! I think we can all agree however that this final hillshade is a much more visually pleasing and a more effective display of terrain compared to that initial greyscale image. It looks less like a photo taken of the harsh lunar landscape and more like an earthscape. So have some fun with it and don’t always except the default values of things. And terrain displayed in this way lends itself well to overlaying other information:
Leave your comments below and, as always, best wishes and happy geoprocessing.