[GRASS-user] How to detect/remove overlapping polygons (imported with v.in.ogr )based on the data attributes

[Veronica Andreo]

[adding grass-user back]

Please, always keep the list when responding

I'm not sure I understand what you mean, but you can see what 2 or n
categories an overlapping polygon has by displaying categories and
centroids from layer 2 in d.vect. Layer 2 is usually where GRASS will store
these topology errors after import.

[image: image.png]

[image: image.png]
HTH,
Vero

El mar, 22 feb 2022 a las 14:56, B H (<hemantbist at gmail.com>) escribió:

> Hi Veronica,
> My understanding (I am not advanced user) that -c will also prevent any
> other cleaning of polygons imported from shapefile.
> There are some small errors in the data that are corrected by v.in.ogr.
> These are the overlaps well above the "snap" threshold I use to import data.
>
> (I may be completely wrong here and would love to correct my
> understanding).
> I think that the two overlapping polygons are represented in Grass GIS
> using a single area with the unique centroid (and multiple category ids).
> I was hoping that grass has a way of associating  and exposing to user a
> unique "centroid id" for each centroid (So I know that two overlapping
> parts are from same area).
> thanks,
> Hemant Bist.
>
>
>
>
> On Mon, Feb 21, 2022 at 4:27 AM Veronica Andreo <veroandreo at gmail.com>
> wrote:
>
>> Hello Hemant,
>>
>> The discussion continued in the thread you linked (see next messages in
>> the thread). I used the -c flag of v.in.ogr to avoid cleaning polygons when
>> importing the shapefile as I needed the simple features to extract zonal
>> stats from raster maps afterwards. Would that be useful to you too in this
>> case?
>>
>> Vero
>>
>>
>>
>>
>>
>> El sáb, 19 feb 2022 a las 0:23, B H (<hemantbist at gmail.com>) escribió:
>>
>>> I have data  in a Shapefile which has genuine  cases of overlapping
>>> geometries. (As documented in  v.in.ogr) the common areas appear with
>>> multiple categories.
>>> I need to find these common areas and "dedupe" them.
>>> Currently I  export the data out via v.out.ogr and in postgis do  select
>>> distinct on (st_asbinary(geometrycomlum))  from tablename  order by
>>> priorityattribute
>>>
>>> Is there some way to either dedupe it in grass Gis itself, or have
>>> v.out.ogr output  some common attribute in the two geometries that
>>> represent the common area
>>> from two polygons.
>>> There is a similar question in this old thread. However I could not find
>>> the solution.
>>> https://lists.osgeo.org/pipermail/grass-user/2019-May/080293.html
>>>
>>>
>>>
>>>
>>> ---
>>> clip from v.in.ogr  man page describing how  the overlapping polygons
>>> are handled.
>>>
>>> Overlapping polygons
>>> When importing overlapping polygons, the overlapping parts will become
>>> new areas with multiple categories, one unique category for each original
>>> polygon. An original polygon will thus be converted to multiple areas with
>>> the same shared category. These multiple areas will therefore also link to
>>> the same entry in the attribute table. A single category value may thus
>>> refer to multiple non-overlapping areas which together represent the
>>> original polygon overlapping with another polygon. The original polygon can
>>> be recovered by using v.extract with the desired category value or where
>>> statement and the -d flag to dissolve common boundaries
>>> Thanks,
>>> Hemant
>>>
>>> _______________________________________________
>>> grass-user mailing list
>>> grass-user at lists.osgeo.org
>>> https://lists.osgeo.org/mailman/listinfo/grass-user
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20220222/fe589cea/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image.png
Type: image/png
Size: 77690 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20220222/fe589cea/attachment-0002.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image.png
Type: image/png
Size: 69019 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20220222/fe589cea/attachment-0003.png>