Geographic boundaries in GRASS raster/vector files
rshepard at appl-ecosys.com
Fri Apr 7 22:33:32 EDT 2000
I've read the programmer's manual (4.2) over and over, and I still can't
answer my own question. So, I'll let you smarter folks answer it for me. :-)
The layer boundary is defined by four values: "north", "east", "south" and
"west". The values (represented in the documents and the spearfish data set)
are in meters and represent the geographic position of the outside edges of
the cells (not the center). That I understand. However, GRASS originally
developed in a simple world where all data sets were projected in UTM. This
results in nice, rectangular map layers with perfectly horizontal and
However, almost all of our work is in State Plane Coordinates. As a matter
of fact, several regulatory agencies specify this projection. Well, SPC
results in a layer which is not orthoginal to the sides of the monitor in
which it is displayed. For example, the upper left corner has a northing
value less than that of the upper right corner (same for the lower corners).
At the same time, the upper left corner has an easting with a lesser value
than the lower left corner.
Given this lack of uniformity in values, how to I enter values when
asked by the program? In other GIS software, the bounds are represented by
the (easting, northing) values of the corners, not the edges. Wouldn't it be
nice if GRASS understood corners, too?
Dr. Richard B. Shepard, President
Applied Ecosystem Services, Inc. (TM)
Making environmentally-responsible mining happen. (SM)
2404 SW 22nd Street | Troutdale, OR 97060-1247 | U.S.A.
+ 1 503-667-4517 (voice) | + 1 503-667-8863 (fax) | rshepard at appl-ecosys.com
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