[Xastir] All DRGs are not always the same!

[Tom Russo]

On Tue, Dec 11, 2007 at 10:32:20AM -0600, we recorded a bogon-computron collision of the <RiverRidge at CenturyTel.net> flavor, containing:
> If you are in Wisconsin and looking for USGS topo maps for Xastir, you 
> might happen on the Wisconsin DNR website.  There they have a repository of 
> Enhanced DRGs.  These maps DO NOT display in Xastir ( or at least I was not 
> able to get to display <grin>).  The problem is exacerbated by the fact 
> that they retain the USGS DRG names for the  "enhanced" version, BUT THEY 
> ARE DIFFERENT!!!

That's because they have been reprocessed with Arc/Info, and as such are just 
"TIFF" files with a couple extra tags, not proper "GeoTiff" files.

See README.MAPS under the heading "Defective GeoTiff files need more special
processing."  The files from Wisconsin DNR even bear the tell-tale tag 
mentioned in that section as a clue that the files are defective:
  Metadata:
  TIFFTAG_SOFTWARE=Arc/Info

The problem is that the Wisconsin DNR DRGs have no tags indicating what
projected coordinate system information is in use, and therefore Xastir 
doesn't know what to do with them.  The other problem is that the files have 
been reprojected into a coordinate system that Xastir couldn't use even if the 
GeoTIFF tags told what system was in use.

In the one case that I downloaded, the FGD file was perfectly valid and the 
.tif file was simply lacking the coordinate system.   Further, Wisconsin
provides its maps warped to the Wisconsin Transverse Mercator system (1983),
not UTM as suggested in the FGD file.

To make it work, one needs to warp the file with the right source reference
system to a projection that we actually can use in xastir.

The EPSG number for WTM83 is EPSG 3070, so:
   gdalwarp -s_srs EPSG:3070 -t_srs EPSG:26715 o44090d1.tif o44090d1.tif2
creates a properly georeferenced file in UTM Zone 15, NAD27 (which was what
it was originally provided in).

Once warped, the files are usable.  The FGD file, in conjunction with the 
warped and georeferenced GeoTiff file, clips the collar properly and the map
is placed in xastir Just Fine.

Without this processing, the files are, as you note, useless.

> Another source of free USGS DRGs that do display properly can be found at 
> www.libremap.org on the Libre Map Project.

Or you can just find files that haven't been mangled beyond usability.

-- 
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
"And, isn't sanity really just a one-trick pony anyway? I mean all you get is
 one trick, rational thinking, but when you're good and crazy, oooh, oooh,
 oooh, the sky is the limit!"  --- The Tick