[Xastir] data in .fgd file for geotiff map

[Tom Russo]

On Tue, Apr 06, 2004 at 04:44:05PM -0400, a Mr. Richard Feyler of Fort Lee, New Jersey <wendell at adsi-m4.com> writes 'Dear Rosanne Rosannadanna':
> I'm using FAA sectionals from
>   http://aviationtoolbox.org/raw_data/FAA_sectionals/current/
> which are (rather large) GeoTiff files.  I have them as maps to
> xastir, and the longitude seems to be right on, but the latitude
> seems to be about 1.4 minutes off.
> 
> The site these are from has a .tfw file (for each .tif file)
> that contains
>   63.753925000000002
>   0.0
>   0.0
>   -63.753925000000002
>   -378078.33973611565
>   -18298.274814258024
> (there are no .fgd files at the site).  Can this be converted to
> the .fgd file?

The tfw file is not used by xastir, it contains details of how the pixel
coordinates of the image are mapped to real-world-coordinates.  It describes
the rotation, translation and scaling needed to do the transformation.  It
duplicates the information in the geotif headers.

> It looks nothing like what mapfgd.pl produced for the map, which
> is
>   1.5.1.1   WEST BOUNDING COORDINATE:  0.000000
>   1.5.1.2   EAST BOUNDING COORDINATE:  0.125000
>   1.5.1.3   NORTH BOUNDING COORDINATE:  0.000000
>   1.5.1.4   SOUTH BOUNDING COORDINATE:  -0.125000
> so I suppose it is something in this file that is incorrect.

mapfgd.pl only works on files that conform to the USGS file naming conventions
(and contain USGS topo maps).  You'll get nonsense out of it on these files
(as you noticed).

If you replace the last field of each of these lines with the actual longitude
and latitude of the bounding coordinates (where the map neatline is) then 
as long as the geotif file has correct metadata in it Xastir should be able
to strip out the map collar.

You might be able to read this information right off of the map collar.

> Or, is there some number I can copy out of the listgeo output to
> put into the .fgd file?

The bounding rectangle of the useful portion of the map isn't contained in 
geotiff headers, hence the need for an fgd file.

> Also, the chart legend and borders appear in the xastir window,
> I suppose I'll need to get rid of those also.

Stripping out this information is all the fgd file is used for.

-- 
Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236 AHTB#1 http://www.qsl.net/~km5vy/
 "It is inhumane in my opinion to force people who have a genuine
  medical need for coffee to wait in line behind people who apparently
  view it as some kind of recreational activity."  -- Dave Barry