[GRASS5] Q. How do you find the islands for a given area?

Rich Shepard rshepard at appl-ecosys.com
Sun Feb 25 12:25:25 EST 2001

On Sun, 25 Feb 2001, Eric G. Miller wrote:

> The biggest killer is the search for the closest tie point between the
> outer polygon and it's inner islands.


  Do you know which objects are islands in which bounding polygons? If so,
my suggestion of a short time ago can probably be modified to comparing the
minimum x values or maximum x values of the two polygons. Then look at the y

  If the relationships among polygons are not known, I wonder if the "plane
sweep" algorithm will help. This algorithm sweeps from max(y) of a polygon
through min(y), stopping at every node's y value and checking for
intersections of this sweep line with polygon edges. Because we cannot
assume that interior holes line up with the nodes of the bounding polygon,
the algorithm can be modified to step down a fixed distance each time
through the loop, and see if there are more than two polygon edges
intersecting at that distance.

  Am I making sense here?


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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