Sun, 13 Mar 2005

Color Ortho Quads in NY

I recently found out that (somebody) created color orthographic quads of New York State. Look for your county on the NYS GIS site. Naturally, instead of publishing them in a lossless open format, they're published in the proprietary MrSID format. This is a wavelet form of compression similar to JPEG2000. There's a decoder called mrsiddecode which creates .tiff or .jpg files as you wish.

I'm in St. Lawrence County (below left). They have complete coverage for 60cmpp colorized infrared (cir) quads (below middle). Unfortunately, they only have partial coverage for 30cmpp color quads (below right). For St. Lawrence County, they only cover Potsdam, Canton, Massena and Ogdensburgh. A friend of mine, Simon St.Laurent, is another map geek. He lives in Dryden, NY, located in Tompkins County. I notice that Tompkins only has 30cm cir and 20cm cir coverage, and no color quads at all. So apparently the phrase "your mileage may vary" applies in spades.

In order to use those images, I turned them into 200x200 pixel tiles similar to those published by Terraserver. Those tiles get thrown into an in-filesystem database which is a sparse local copy of Terraserver. Whenever any of my mapping software fetches a map from terraserver, it populates the database with it. I've only published pygps and mapview. I haven't yet published maptracks (makes a map with a GPS track overlaid), make-tiles (which splits up the color ortho quads), nor make-tiles-index (which creates the coverage maps above). They need improvement before they're seriously usable.

These datasets get very large, by the way. I've recently discovered the magic of external hard drives using USB 2.0. I picked up a 120GB Western Digital drive from Office Max for $60. The ortho quads amount to 3.9GB, but if you uncompress them all to .tiffs, you'll fill up all 120GB. I need to uncompress on the fly with make-tiles.

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