[Xastir] Bounding boxes [Re: USGS maps no longer free]

[Tom Russo]

On Mon, Oct 06, 2003 at 08:05:46PM -0400, a Mr. Richard Feyler of Fort Lee, New Jersey <hacker at tc.fluke.com> writes 'Dear Rosanne Rosannadanna':
> On Mon, 6 Oct 2003, Tyler Allison wrote:
> 
> > How do I know which number to use as the east/south/west/north boundaries?
> > The listgeo file has two numbers listed for things like 'lower left' or
> > 'upper right' which obviously isnt east or south ;)
> 
> Pick the average?

I wouldn't.  I would pick the most westerly of the two left corners as
the west boundary, the most easterly of the two right corners as the
east, etc.  That'll guarantee that a rectangle made up of N-S and E-W
lines will entirely enclose the data.  If you pick the average and the
map isn't actually aligned with N/S,E/W as this one appears not to be,
then if the FGD file is used to clip the boundary you'll lose bits of
the map.  But picking them this way will include bits of the collar, too.

But I'm concerned about the actual values in Tyler's examples --- USGS
maps are usually defined by very simple lat/lons --- for example, the
7.5-minute quads are aligned so that they have west boundaries of,
say, 106d-105d52m30s for one map, 105d52m30s-105d45m for the next one
to the east, 105d45m-105d37m30s for the next, etc.  That the
re-projected corner points don't map to neat values concerns me ---
perhaps they were projected back using the wrong datum?  The
difference seems too great for that, though.

There's a naming trick that USGS used, and that's exploited by
mapfgd.pl in the scripts directory --- if you got them from USGS and
haven't changed the name, maybe you could use that?

-- 
Tom Russo   KM5VY    QRPL #1592   K2#398    http://www.swcp.com/~russo/
Tijeras, NM DM64ux   SOC #236     AHTB#1    http://www.qsl.net/~km5vy/
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