LiDAR in Belize
There’s an interesting open-access feature in the latest issue of Archaeology — which also has a nice shot of a tomb at Madain Salih on the cover — about using aerial LiDAR to map features hidden under jungle canopy around Caracol in Belize. (“That’s great,” I hear you say, “but is there anything in there about the Donner Party’s dog?” I’m glad you asked, because yes, there’s this.) This story isn’t really new — it’s appeared in several other places recently — but I read about it then, forgot about it, and then read this as I was glancing at the Archaeology site a few days ago, and thought I’d mention it here.
The interesting thing about this for me is that, when I was first told about it by one of the undergraduates (now a former undergraduate) working in our lab, it seemed impossible. The active sensors that tend to be good at “seeing through” things like leaves (or sand) are in the low-frequency, large wavelength part of the spectrum, like L-band SAR (Synthetic Aperture Radar), which has been used to do exactly that. Visible light, on the other hand, is much higher-frequency, and not so good at penetrating dense tree cover. From a common-sense perspective, this shouldn’t be terribly surprising; it’s darker under dense canopy than it would be if you were standing on top of the trees, because visible light doesn’t make it through. So, it seems like what you would get if you took scans of an area of dense canopy would be a very nice image of all of those trees. And this is, in fact, exactly what you get (see the “b” view of this National Geographic image, for example).
But somehow they did actually manage to map what was under there, and I think the way it works is pretty neat. As the authors put it:
Initially, the lasers are refracted by the tops of trees, producing a detailed record of the forest cover. But treetops are porous, so some photons penetrate deeper, while others reach all the way to the ground and reflect back from the underlying surface terrain—and any buildings or ancient structures on it. The result is an accurate, three-dimensional map of both the forest canopy and the ground elevation beneath it.
This was my guess as to the only way it could work when I heard about the story last month, and as it turns out, that is the way it works. Again, it makes sense from the common-sense perspective. The only way that an active sensor in the visible spectrum could penetrate forest cover is the same way that light normally penetrates forest cover: through all the little breaks between leaves. After a quick Googling, I fount out that this isn’t an entirely new idea, either, although it was the first I’ve heard about anyone using LiDAR this way. Its application to archaeological survey still seems rather novel, though, and productive for areas where pedestrian survey is made difficult by forest density. And it’s always nice to see aerial remote sensing being used successfully to make accurate archaeological maps.
2010 Chase, Arlen F., Diane Z. Chase, and John F. Weishampel Lasers in the Jungle. Archaeology 63(4). http://www.archaeology.org/1007/etc/caracol.html.