[GRASS-dev] [GRASS GIS] #3030: v. generalize fails to generalize polygons in a random manner

GRASS GIS trac at osgeo.org
Wed May 11 13:26:15 PDT 2016

#3030: v. generalize fails to generalize polygons in a random manner
  Reporter:  dido    |      Owner:  grass-dev@…
      Type:  defect  |     Status:  new
  Priority:  major   |  Milestone:  7.0.5
 Component:  Vector  |    Version:  7.0.4
Resolution:          |   Keywords:  v.generalize
       CPU:  x86-64  |   Platform:  MSWindows 7

Comment (by mmetz):

 Replying to [comment:10 dido]:
 > OK, I get what's breaking up generalization.
 > Talking about test1.png - it's a piece of polygon boundary that's
 several hundred meters in length, unfortunately causing a self-
 intersection. Why v.generalize discards the whole polygon side that's
 several kilometers long but not the problematic sector only?

 Because implementing such a feature would slow down v.generalize
 considerably, it is already rather slow for complex vectors. Splicing in
 an original part of the boundary into the modified version could also
 cause quite a few other problems (no common vertex to start with), and
 topological correctness is not guaranteed. Note that boundaries and
 polygons are not the same.
 > Actually in my first tryout the whole area was not fishnet'ed to
 5000x5000 m cells so  small problematic sectors resulted to tens and
 hundreds of kilometers of shoreline being not generalized at all, making
 the tool completely unpredictable in the output.

 As I suggested previously, try stepwise generalization, starting with a
 small threshold, then increasing the threshold and using the output of the
 previous run as input for the next run. In your case, a threshold of 120
 seems rather large considering the vertex density of the boundaries.

 I suggest to close the ticket as invalid because v.generalize correctly
 discards boundary modifications that would result in errors.

Ticket URL: <http://trac.osgeo.org/grass/ticket/3030#comment:11>
GRASS GIS <https://grass.osgeo.org>

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