nps!susan at cerl.cecer.army.mil
Tue Mar 23 10:39:13 EST 1993
>I have a problem when I am running v.support for
>a vector file that I have imported using v.in.ascii.
>There are two adjacent polygons in the file which share about
>four points. The coordinates in the attribute file definitely
>fall within each polygon. When I run v.support, I get the
>PNT_TO_AREA FAILED ...
>for both polygons and attributes. However, if I create two
>separate ASCII vector files each with one polygon and one
>attribute, I can successfully run v.in.ascii and v.support.
>Has anyone had a similar experience or know why this is
>happening? The points that the polygons share are identically
>the same in each polygon list.
>College of Forest Resources phone: (206) 543-5506
>University of Washington AR-10 FAX: (206) 685-3091
>Seattle, Washington 98195 e-mail: bac at hardy.u.washington.edu
It sounds as though you have two complete polygons in your vector file,
GRASS wants its vector data in an arc-node format. There needs to be a
node where ever three lines (arcs) come together, that is anywhere two
polygons' outlines abut, there should be one and only one copy of the
edge defining the seperation of the two polygons. That means you need to
take your file into v.digit, break the polygon outlines at the point of
intersection(s), remove the duplicate line between the two polygons,
resnap any lines to nodes if necessary, and then run v.support.
GRASS is somewhat forgiving of complete polygon definitions
except where the polygons both start at the same point. Then it doesn't
know how to build the individual outlines. You might try the new release of
v.spag in 4.1, it should be able to deal with your data set automatically
rather than spend enormous time trying to edit by hand, assuming that you
have more data than just these two polygons...
National Park Service
Geographic Information Systems Division
PO Box 25287 12795 W. Alameda Parkway
Denver Colorado 80225-0287 Lakewood Colorado 80228
susan%nps at cerl.cecer.army.mil
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