[Gdal-dev] trouble converting units in raster images

[Jeff Hoffmann]

Jeff Hoffmann wrote:
> I have a raster layer that is composed of a few hundred GeoTIFF files; 
> all of the files are NAD83 / Alabama West, although some have the units 
> set to meters, other are feet.  I'm trying to get them all to the same 
> units, but I can't figure out how to do that.  I'm trying to do it with 
> gdalwarp (with parameters "-s_srs '+init=epsg:26930 +units=ft' -t_srs 
> '+init=epsg:26930'").  I'm using GDAL version 1.3.1.0.  The command 
> changes the coordinate system information when I do a gdalinfo on the 
> file, but the origin, pixel size, corner coordinates all stay the same 
> numbers.  Is there something I'm not doing right?  It seems like it 
> should be pretty straightforward.  Or is there some other way to 
> accomplish this if it just won't work?

OK, I feel a little stupid now, but also a little confused.  To get this 
to work, I removed the "-s_srs '+init=epsg:26930 +units=ft'" and just 
used the "-t_srs '+init=epsg:26930'".  I don't know why that worked 
because the source SRS parameter should match what was in the file, but 
apparently something must be different.  That's not the confusing part, 
though.  I tried a bunch of different parameter combinations and 
sometimes I would delete the existing target file and sometimes I 
wouldn't.  It would just happily run if there was an existing target 
file (or at least it didn't give me an error) but the corner 
coordinates/pixel size always stayed the same even though the units 
changed (the coordinate system information did get updated from ft->m).

-- 
Jeff Hoffmann
jeff at propertykey.com