Mapping the Forest and the Trees: A Visit With Remote Sensing Expert Alan Strahler

Prof. Alan Strahler's current work involves a ground-based LiDAR instrument called ECHIDNA, after the spiky Australian egg-laying mammal.
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Events of January 18, 2012

I'm always on the lookout for space stuff, and I've found it in some pretty unusual places, like the space shuttle launch photo that was in a Prada sunglasses ad the last time I visited the optometrist. Over the holiday break, I saw an article in the online version of Bostonia magazine reporting that Alan Strahler, a professor here at Boston University, had received NASA's prestigious William T. Pecora award for his work in remote sensing. As my long-time readers will know, I hope to become a remote sensing archeologist someday, using satellite images to reveal hidden traces of past human activities and new archeological sites, so I was eager to speak with Professor Strahler. I arranged to visit his office on Wednesday afternoon, the second day of the Spring term.

The College of Arts and Sciences building is pretty big and a little confusing, but luckily, Professor Strahler's office was in a part of the building I was very familiar with -- it was right across the hall from Farouk El-Baz's office, which I'd visited on the first day of school.

Professor Strahler was glad to meet me, and apologized that he didn't have any experience in the use of remote sensing technology for archeology -- as a member of the Geography and Environment department, his own research focused on the use of satellite data to study vegetation and land quality. However, there was still a lot he could tell me about remote sensing technology in general.

First of all, he explained, all satellites used by scientists to study the Earth are NOT the same -- some can cover the whole Earth over the course of a day, while others can deliver zoomed-in views of specific areas of interest. The later might be ideal for observing a volcano in the process of erupting, while the former could reveal where global wind patterns are carrying the clouds of ash and where air travel might be affected. LiDAR, or Light Detection And Ranging, is a special technique that maps changes in the elevation of a region by bouncing laser light off a target and measuring how long it takes the light to return -- it's a bit like underwater sonar, except that the beam is narrower. And, because different substances reflect the light more than others, LiDAR can also tell scientists the composition of the terrain being studied. (They can even determine if snow on the ground is new or old -- dirtier, older snow is less reflective than new-fallen snow.)

Professor Strahler started off creating maps based on Landsat data before moving on to work with NASA's Terra and Aqua environmental satellites. Because these satellites carry identical MODIS (Moderate Resolution Imaging Spectroradiometer) instruments and have highly coordinated orbits, they can provide cloud-free data about the whole Earth on a day-to-day basis.

His current work involves a ground-based LiDAR instrument called ECHIDNA, after the spiky Australian egg-laying mammal. (The kind of animal Knuckles is supposed to be in Sonic the Hedgehog.) Just like an echidna is protected by spines that stick out in all directions, the ECHIDNA instrument shoots out its lasers at every possible angle to map the environment around and above it. By combining all of the data points, the scientists are able to make three-dimensional maps of areas of forest, including their average height and width.

I asked Professor Strahler how the data was represented -- since the map is of roughly half a sphere, I thought it might look like maps of the world as seen from the North or South Poles. He said that some of their images were like that, while others showed the data as if it were on the inside of a cylinder. (You can see both kinds of images here.)

However, he said, there was a much more dynamic way that they sometimes represented the data -- as a three-dimensional animation that you can move around inside of and change your viewpoint, like an environment in a video game. He showed me simulated flythroughs of a giant sequoia forest and a forest of smaller trees created using data from the ECHIDNA on his computer. The difference in size between the trees was very apparent -- I can see why they're called GIANT sequoias!

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