#754: Deep Vector Bug: Overlay functionality flawed in v.select and v.in.ogr
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Reporter: ploewe | Owner: grass-dev at lists.osgeo.org
Type: defect | Status: new
Priority: critical | Milestone: 6.5.0
Component: Vector | Version: 6.4.0 RCs
Resolution: | Keywords: v.overlay
Platform: Linux | Cpu: Unspecified
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Comment (by mmetz):
Replying to [comment:3 ploewe]:
> Replying to [comment:2 mmetz]:
> > Replying to [comment:1 martinl]:
> > > > Sample data / Screenshots can be produced on request.
> > >
> > > Yes, please attach sample data e.g., in GRASS ASCII vector format.
> > >
> > +1, if the sample data are too large for attachment, please send them
off-list as attachment.
> >
> > Could you also provide information about what were the exact commands
you used, what do you want to happen with the overlapping areas i.e. which
cleaning operations should be performed, and what should the desired
result look like.
> >
>
> Done.
> Two sample input data sets were attached as ESR! shapes to the ticket
plus the result from v.patch as generated by grass6.4R5.
> Here's the challenge: The two tile grids provided in the two input
shapefiles partially overlap. The task is NOT to clean the topology (and
to create new sub-polygons in the overlap area), BUT to maintain the
current polygon pattern, which implies that a number of poylgons overlap.
AFAICT, this is not supported by the GRASS vector topology model by
design. In order to attach a centroid to an area, at least some part of
that area must not be overlapped by any other area, otherwise there will
be areas without centroids and/or areas with duplicate centroids. In your
patched example, there are a number of areas that are completely
overlapped by other areas. There, centroids can neither be calculated nor
properly attached. Patching these two vectors together and maintaining
areas exactly as they are, with their category and attribute link, works
only in a non-topological vector environment.
Partially overlapping polygons are supported by GRASS, e.g. Landsat tiles
can be imported and maintained as they are if they are not cleaned,
because tiles overlap only at the edges and centroids can be properly
attached. As long as there is a small bit of the area not overlapped by
another area, you can manually move the centroid to there to remove
duplicate centroids. This is however impossible for a number of areas in
the overlap zone.
Markus M
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Ticket URL: <https://trac.osgeo.org/grass/ticket/754#comment:4>
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