[GRASS-user] importing and cleaning overlapping polygons that are supposed to overlap

[Veronica Andreo]

Hi Markus,

Thanks for the explanation. It is possible to import topologically
incorrect vectors, but not create them within GRASS. So, as a solution to
my problem, I rather create (potentially overlapping) buffers outside GRASS
and import them with -c in v.in.ogr to use v.rast.stats with no loss of
areas, or follow the procedure that you described earlier in the thread.

Cheers,
Vero

El mié., 22 may. 2019 a las 18:48, Markus Metz (<
markus.metz.giswork at gmail.com>) escribió:

>
>
> On Wed, May 22, 2019 at 3:11 PM Veronica Andreo <veroandreo at gmail.com>
> wrote:
> >
> > Dear all,
> >
> > thanks again for your answers. I found an easier way, use -c flag in
> v.in.ogr seems to not build topology of overlapping areas and then
> v.rast.stats does not complain.
> >
> > However, if one builds buffer areas for points within GRASS, i.e., using
> v.buffer, the problem appears again when buffer areas overlap, which it's
> pretty common when creating buffers around neighbouring points. Then, to
> get zonal stats for those areas the approach suggested by Markus Metz
> should be followed. I believe it is a bit of an overkill for such a common
> task in GIS. Couldn't v.buffer have a -c flag as v.in.ogr so when
> overlapping of buffer areas is fine the topology is not built and one would
> get just one area per input point? Would that be possible?
>
> In short, no.
>
> The reason is that with a topological vector model, areas are constructed
> from boundaries and centroids. If you use the -c flag of v.in.ogr and there
> are polygons that overlap to a large degree or completely, it is not
> possible to find a centroid for each topological area, i.e. the overlapping
> input polygons can not be properly represented with a topological model
> when using the -c flag. As soon as you get incorrect boundaries and/or
> duplicate centroids when building topology, results are wrong.
>
> Markus M
>
> >
> > cheers,
> > Vero
> >
> >
> >
> > El jue., 16 may. 2019 a las 11:27, Moritz Lennert (<
> mlennert at club.worldonline.be>) escribió:
> >>
> >> On 16/05/19 03:11, Veronica Andreo wrote:
> >> > Dear all,
> >> >
> >> > thanks for the answers...
> >> >
> >> > @Madi, I know, but that's how I got the data from a colleague using
> >> > SaTScan to get cluster sizes.
> >> >
> >> > So, these "clusters" are 3, they are represented by circular areas
> and 2
> >> > of them overlap, and it is just fine that they overlap. I just want
> one
> >> > centroid per area to be able to get one value per original area.
> >> >
> >> > I tested your solution, @Micha (thanks much for your time!), but it
> >> > gives me 4 values, and I need 3. Moreover, I can no longer recognize
> >> > which polygon is which since v.db.select for layer 3 reports cats
> from 1
> >> > to 4, but d.vect shows something different (I'd assume 2/3 has become
> >> > 4?). See screenshot below.
> >>
> >> To see the cat values of layer 3 you have to indicate that layer in the
> >> label_layer parameter (Labels tab).
> >>
> >> You can also see the correspondance between the two layers with
> >>
> >> v.category polys_3layers op=print layer=1,3
> >>
> >> >
> >> > So, is it possible somehow to keep my 3 original polygons? And how
> can I
> >> > get raster values for my original 3 polygons in GRASS? Or is it that
> >> > this is not allowed by topology?
> >>
> >> This depends on how you want to get raster values. If v.what.rast at the
> >> location of the centroid is all you need than Micha's solution should
> work.
> >>
> >> If you need v.rast.stats then you either have to use Markus' suggestion,
> >> or you use Micha's approach running v.rast.stats on layer three and then
> >> use some magic to transform the output of the above v.category call to a
> >> correspondance table between layer. Something like this
> >>
> >> import grass.script as g
> >> for feature in g.read_command('v.category',
> >>                               input_='polys_3layers',
> >>                               op='print',
> >>                                layer=[3,1]).splitlines():
> >>       cat1 = feature.split('|')[0]
> >>       cats2 = feature.split('|')[1].split('/')
> >>       for cat2 in cats2:
> >>           print cat1, cat2
> >>
> >> This correspondance table could then be used to aggregate the raster
> >> stats per (original) polygon in SQL. This could be the easily put into
> >> an addon module.
> >>
> >> Moritz
> >>
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>
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