On Sun, Oct 26, 2008 at 06:05:54PM -0500, we recorded a bogon-computron collision of the <[EMAIL PROTECTED]> flavor, containing: > Thanks, Tom.......... > > I guessed you probably had.... but I didn't guess gdal! > > So I (apparently) successfully installed the gdal utilities. For an > example in the documentation, they show: > gdal_contour -a elev dem.tif contour.shp -i 10.0 > > Is that a good set of options?
Depends. The interval is in whatever units the DEM is in. If that's in feet (as some are) then you get 10 foot intervals with those options, which might not be what you want. > Don't you need to put in a base > elevation or something? Not really necessary, but you can. With the arguments in the example command line, it'll generate contours at 0,10,20,30, etc. And so if your DEM only has values between 5000 and 10000, you'll get only 5000,5010,5020,etc. You can specify a base elevation with the "-off" option, and it'll start counting from there. Otherwise it starts counting from 0. > Are the contour lines labels with elevation? Only if you specify the "-a <name>" option to give it an attribute name. The example command line has "-a elev" so the shapefile will have an attribute called "elev" with the elevation of that line. >I > don't understand the -3d, -inodata, or -snodata.... they are not needed > for the typical USGS dem? You can probably ignore most of those. Raster data, since it is always rectangular but might only have legitimate values for some other shape (say, a rotated rectangle or other distorted outline), must have some value to represent the fact that some pixel has no valid data. If you do a gdalinfo on the DEM, you'll see that the DEM format has information in it that tells GDAL what the "no data" value is, so you needn't specify it. "-3d" will create a 3-D shapefile with X,Y, and Z values at each point instead of just X,Y. For Xastir, you don't need that (and it is only relatively recently that xastir didn't just core dump when given a 3-d shapefile). > Any other clues as my DEM downloads? Nope. Just make sure you know what units the DEM is in. I've seen 'em with elevations in feet and with elevations in meters. -- Tom Russo KM5VY SAR502 DM64ux http://www.swcp.com/~russo/ Tijeras, NM QRPL#1592 K2#398 SOC#236 AHTB#1 http://kevan.org/brain.cgi?DDTNM "Argue for your limitations and sure enough, they're yours." -- R. Bach _______________________________________________ Xastir mailing list [email protected] http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
