Mar. 17th, 2008

I'm (re-)implementing/packaging up some first steps in my sometime-officemate's cool new spatial cluster detection algorithm. It's graph-theoretical, and we're using some open-source libraries for things like finding Voronoi diagrams of initial (control) points. It's cool stuff, but it's been bothering me a bit--I'm sure that library just calculates Euclidean distances, and we're working with location information. In fact, what we have available is geocoordinates.

I should have known that my mathematician officemate would NEVER just gloss over the distinction between distance on the plane and distances on the Earth's surface. I finally asked her about the input data. "Oh," she said, "I usually translate the geocoordinates into a state plane projection first." (And thank goodness for Google, because she didn't include any details. There are plenty of links out there, though.)

I never knew these things existed. More than you (or I) ever wanted to know about these... )

Off to ask Google if there's any more user- (or at least lehser-) friendly free downloadable code out there for getting state plane projection coordinates out of geocoordinates. Or maybe just decide to use some other, more globally-suitable projection. Or assume that the difference between planar & actual distances won't be too bad at relatively small scales and ignore the whole thing. Though I have to be encouraged by this quote from the link above, on an engineer who wanted to do just that: "(I've always had the impression that this engineer was either lousy with math or just plane lazy, but I'm probably being unfair)."



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