[Xastir] DRG Problem

[Tom Russo]

On Fri, Mar 11, 2005 at 01:26:53PM -0500, we recorded a bogon-computron collision of the <mckeehan at mckeehan.homeip.net> flavor, containing:
> If you go through the process to purchase and select a DRG, you can choose to
> download as opposed to get it on a CD - then the cost goes to zero.
> 
> The specific file that I am looking at can be found here:
> http://mckeehan.homeip.net/xastirmaps/TVADRG/
> 
> The .tar.gz file has both files in it.

You said you had a TFW file --- could you put that up on your site, too, 
please?

The problem is that these tif files are just image files, not GeoTIFF
files --- they have NO geocoding at all.  Xastir will be unable to do anything 
intelligent with them as is.

However, the TFW files contain the parameters necessary to define the 
linear affine transformation from image coordinates to some projected 
coordinate system.  That, combined with the metadata from the original
web site (assuming it's there) should enable you to use gdal tools to 
get correct GeoTIFF tags.  One could (with more work) even use geotifcp.
In fact, the first step is to use geotifcp to attach the world file (tfw)
and tiff image.

To get it right, you have to do two things:  get the worldfile installed
into geotiff tags so that the image->projected coordinate system transformation
is in the file.  Then you have to give some tool (I like gdalwarp) the
information it needs about the projected coordinate system so it can
create a new file with correct geocoding.  If the projected coordinate
system is one with an associated EPSG number then that last step is very, very 
simple.  If not, you have to do more work.  Either way, you need the metadata
for the map file to proceed.

Without the geocoding, though, the tif image is nothing more than a 
pretty picture, and you won't be able to do anything with it in xastir.
(And before anyone says to use a .geo file instead, don't --- odds are very 
good your .tif image is in UTM projection, or maybe even in Lambert Conformal 
Conic or some state-plane system, and is large enough that the errors in using 
the sort of "georeferencing" that is possible with .geo files would be so 
great that the map will display, but be incorrect almost everywhere but at
its corners).

-- 
Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236 AHTB#1 
 "When life gives you lemons, find someone with a paper cut."