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