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Susan Stitt nps!susan at
Tue Mar 23 10:39:13 EST 1993

>I have a problem when I am running for
>a vector file that I have imported using
>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, I get the
>error message
>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 and
>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.
>Thank you,
>Brian Connelly
>College of Forest Resources                  phone:  (206) 543-5506
>University of Washington  AR-10                FAX:  (206) 685-3091
>Seattle, Washington  98195                  e-mail:  bac at

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  

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...  

Susan Stitt

  National Park Service                      
  Geographic Information Systems Division
  PO Box 25287                               12795 W. Alameda Parkway
  Denver Colorado  80225-0287                Lakewood Colorado  80228

  (303) 969-2596
  susan%nps at

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