USGS page like this one.
The web viewer is impossible, so that link over on the left -- Download Image Now (16M) -- looks awfully tempting, and I always go for it.
What they don't tell you is what sort of image you're getting; after you download that 16M, you end up with a file called something like q250_1388a_us_c.sid, which no image viewer I've ever found considers to be an image file. Even ImageMagick, which can handle almost anything, is baffled by .sid files.
It turns out that .sid stands for "Mr. Sid", a file format for very large raster images. The format is controlled by a company called LizardTech, and it's apparently so scary that no one has ever managed to reverse engineer it. The only way to read a Mr. Sid file is to use one of the programs (available in binary form only) from LizardTech.
Fortunately LizardTech does provide at least one of their programs, mrsisddecode, as a Linux binary. Get it from their download page. Then you can type a command like mrsiddecode -i q250_1388a_us_c.sid -o q250_1388a_us_c.jpg to convert the file into some other image format (which will be quite large -- this particular map is 17170 x 9525).
(Apparently there's an SDK which is also available for Linux, available here. The gdal toolkit used by MapServer and certain other GIS applications make use of this SDK. I hear it's somewhat picky about GCC version, but otherwise works.)
I'm happy that I've found something that will convert MrSid files to a format I can use, but it's a little discouraging that the USGS is restricting its public maps to a format that can be read only with software from a single company. I wonder if the USGS has a contingency plan concerning all these Mr. Sid maps in case anything ever happens to LizardTech? Aren't open formats safer in the long run?
[ 21:50 Apr 18, 2006 More science/geology | permalink to this entry ]