Tuesday, January 26, 2010

Geodesic Distance in ArcMap

One of the things I like about ESRI ArcMap is the ease with which you can measure features in your data. Recently I wanted a a simple way to measure geodesic distances of some projected data. The measure tool in ArcMap, by default, measures distances in projected units in projected data, and geodesic distances only for data displayed in geographic lat/long. If you hold down the shift key while measuring a distance, Arc will calculate geodesic distances regardless of projection. What I really wanted to be able to do, however, was to be able to draw lines in a shapefile, and then have Arc calculate the geodesic distance of those lines. This is something that Arc cannot do, without you first unprojecting (or more correctly, reprojecting) your data to geographic coordinates.

Luckily, I found this:

Looks like some of the folks at ESRI got together with the USGS a designed a plug-in that will let you, among other things, calculate geodesic distances from projected data. It uses the parameters specified by your projection to unproject the data into geographic coordinates and then calculates the geodesic distance on the fly. Since it uses the semi-major and semi-minor radii that you specify in your projection, the distortion should be minimized. This tool is definitely a time saver, as it does the reprojection for you only on the data you are calculating, and enables you to keep your whole project in projected space.

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