Contours

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Nelson's log

I wanted to play with contours for some mapping. Ie: a shapefile of where the 8000′ or higher terrain is across the US. Turns out to be pretty simple to generate something basically working from SRTM data. The gdal_contour program does the work. The OpenStreetMap folks have some nice notes on how to use it with SRTM data. The details, however, are much harder.

Basics

$ gdal_contour -a elevation N40W108.hgt N40W108.shp -i 304.8 -snodata 32767

That generates 1000′ contours, very fast, maybe 150ms with the data in cache. The output file size varies wildly: for that hilly part of Colorado it’s 1 meg. Generating 100′ contours instead takes about 6x as long and results in 10x the resulting file size. (gdal_contour is dumb about files: if the output file already exists, the error you get is “ERROR 1: N40W108.shp is not a directory”)

North America has 2415 data files so even on…

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