Many customers ask us why we use aerial photography. Often it’s hard for some readers to understand because of terrain inversion. Let’s take a Q&A with a real customer:
Customer, “Hi guys. I just wonder if anyone has ever discussed with you the fact that the shading on your maps appears to be reversed. That is, the high points appear like low points and vice versa, if one is looking at the map oriented to north. If one turns the map around, so it is oriented to south, then everything looks correct. So, my guess is simply that the shading is on the wrong side of the ridges. ”
Our response, “Thanks for asking a question that has popped up a few times. In the mapping biz, it’s called terrain inversion and it is a problem sustained by tradition. Before aerial photography, most terrains were shaded as if the sun was in the upper-left corner. This was established as cartographic tradition because we read from upper-left to lower-right.
This worked fine until aerial photography became used as base maps. In the northern hemisphere, and especially at our latitude, the sun is never shines from the north. All of our shadows fall from south to north, completely opposite to how they’re depicted on most conventional maps. Cartographers in the southern hemisphere think we’re crazy up here moving the sun around to accommodate tradition at expense of reality. But they have the advantage of sun always being in the upper quadrants, thus giving the type of illumination you’re probably used to.
But we use aerial photography in most of our maps, because it is reality and at the right scale is far superior to an artificial hillshade in our opinion for couple reasons. You can see areas that are really in shadow, which indicate different plant communities. You can see canopy type, density, and other indicators for type of forest. If you’re a photographer, these images show depth and lines of shadow, helping you predict the kind of light you want for a photo. Finally, if you have take an aerial photo up high to orient, you can do so by shadows.
Detail from our Big South Fork map with aerial photography. Note the north facing cliffs are in shadow, just as in found in the wild.
So in my defense, I say it’s all about sunlight. I understand your problems and trust me, it has stirred the anger of my peers at cartography conferences. We’ve been flamed on blogs for this apparent flaw, but in our humble opinion, an aerial photo is not the problem, it’s tradition.
It’s sounds like you have a lot of experience with maps, and I respect your opinion. However, I think if you get used to aerial photography, you’ll find distinct advantages. Also in defense of tradition, I did illuminate our recent map, Trail Atlas of the Great Smokies from the NW. That map does not use aerial photography so it was an easy decision.”
