HELP

mike camann camann at pick.uga.edu
Sun Jan 30 15:31:58 EST 1994


GRASS-netters,

I hate to be a pain about this, but I really am stuck and *nobody* has
replied to my earlier posts, so I'll try one more time.  Would anyone
who has created GRASS databases on the hemispheric scale please contact
me.  Here is the problem:

I need to digitize a map of North and Central America that was produced
by the USGS in 1982.  In order to do so, I have created a $LOCATION
north_america.  The map is a transverse Mercator projection of the
globe from 90N to 5N, 1:10,000,000 scale.  The central parallel is 45N
and the central meridian is 100W with a scaling factor of 0.926.

When I create the $LOCATION, how do I determine the corners of the
region, e.g. at the DEFAULT_WIND input screen?  The tmerc projection
demands that the corners be specified in meters, rather than lat-lon,
so I measured the map edges, converted the measurements into meters on
the ground, and entered the information as:

N=8510000  S=0
E=8770000  W=0

on the assumption that the bottom left corner of my region was
arbitrarily assigned (0,0) coordinates, the region extent was
determined by my northing and easting measurements, and that the window
so defined would be centered over the intersection of the central
parallel and central meridian.  This is evidently NOT the case, since
all attempts to digitize the map fail: the control points that I enter
(in lat-lon coordinates, as printed on the map) give extemely high
residual errors (>2500).  If I accept the residuals and check the
locations of known points, they are off by thousands of meters.  The
same thing happens when I try to register control points measured in
meters relative to the region borders (the edge of the paper map).

My guess is that the region definition is wrong, that the (0,0) point
is actually at some fixed point on the globe (?) and that I need to
define my DEFAULT_WIND relative to that fixed point, as would be the
case if I were able to enter my initial coordinates in lat-lon.  If so,
WHERE is that point?

Alternatively, am I doing something else wrong?

Would ANYONE who has successfully created a $LOCATION covering North
America please contact me so I can find out how you did it.  Is it
*possible* that no one has ever done this (hence the silence after my
previous requests for help)?

Thanks (hopefully...).

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Michael Camann                          camann at dial.pick.uga.edu
Department of Entomology                camann at phoenix.cs.uga.edu
University of Georgia                   (706) 542-1388
Athens, GA 30602                        (706) 542-2276

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