[Xastir-Dev] problem with drg geotiff maps

[Curt Mills, WE7U]

On Thu, 31 Oct 2002, J. Lance Cotton wrote:

> I was playing with the .fgd files from these DRG-E's. I know that they
> are 7.5 minute 'windows', and the bounds in the FGD don't match with
> that. I assume that's because they have been re-projected in some sort
> of rectangular something-or-other. I'm not a GIS person, I just like to
> look at maps :-)

The did something that I think is very screwy:  They added portions
of the nearby maps to the map edges in order to make them
rectangular.  This makes them fit together nicely edge-to-edge, but
think about a single station traveling through that area:  At some
point the station would be shown in two spots, where the map areas
are duplicated, then would join back into one as it drove out of
that area.

North of the equator UTM projection maps (which is what DRG and
DRG-E claim to be) are narrower at the top than the bottom.  South
of the equator it'd be the other way around.  The tilt increases as
you get farther away from the equator in either direction.

The area along the map edges is small in Texas, but it gets much
bigger as you go farther north.  I don't like the technique they
used to make the DRG-E maps rectangular.  I can imagine SAR missions
where we're working in that border area and have problems trying to
figure out positions on the topo maps.  I need to look closer at
that area in our state (WA) to see just how wide of an area it is.
It may still be small enough as to be inconsequential.

Xastir stretches the map into a rectangle as it draws it, so that we
don't have that problem at all.

-- 
Curt Mills, WE7U                    hacker_NO_SPAM_ at tc.fluke.com
Senior Methods Engineer/SysAdmin
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