Tom's Blog

Tuesday, November 11, 2008

Using GIS to compare air photos

I am slowly learning the capabilities of ArcGIS, the widely used mapping software that has elements of database and graphics arts. I have been able to bring in two air photos of Pleasant Valley Conservancy, one from 1937 (grayscale) and the other from 2007 (color). There are some important contrasts, but overall the two photos look surprisingly similar. Mainly, this similarity reflects all the restoration work we have done over the past 10 years, because if I showed a 1997 air photo it would look completely different.

Of course I cheated a little, since I carefully cropped them in Photoshop so that their boundaries were identical.

In ArcGIS I can layer one of these photos on top of the other, change the transparency of the one on top so that the bottom one shows through, and overlay with symbols representing trees from of our tree database. I can also measure areas and distances. For instance, the cropped area that is now Toby's Prairie (the open field at the top right) was 7.6 acres in 1937, and is 3.6 acres in 2007 . What happened to the rest? Actually it is now part of the savanna to the south, across the service road that can be seen (faintly at this scale) on the color photo. I believe that once they started farming with tractors instead of horses, they were unable to crop down the hill as far, in fear of getting stuck.

Another thing I have been able to do with ArcGIS is bring in a bedrock map of Dane County (kindly sent to me as an ArcGIS shape file by the Wisconsin Geological and Natural History Survey). This has made it possible for me to map exactly where the dolomitic rocks are found, which is helping me understand the distribution of buckthorn (since buckthorn is a calciphile found almost exclusively at Pleasant Valley on the dolomitic soils).

Since I have a whole set of air photos of our property, I should be able to create an interesting time series, providing lots of quantitative data that was not previously available. More to come!


Blogger Emily said...

this is really cool Dad! Can't wait to see some images of the results showing these differences.

November 11, 2008 at 7:50 PM  

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