[Qgis-user] Removal of overlap geometries within the same layer - merge to largest neighbor polygon

[Andreas Neumann]

Hi,

Thank you all for your feedback!

@Nyall - the GEOS/JTS capabilities for cleaning and validating geometries
look definitely interesting. Something for an upcoming QGIS-CH grant
perhaps ...

@Bernhard - I also had a look at and tested the "Eliminate selected
polygons" algorithm - but it doesn't have a way to specify attribute
handling nor grouping - and it is also strange that I have to select the
polygons first and cannot run the algorithm on the whole data set as is

About SAGA: I couldn't get the SAGA stuff to work anymore
About GRASS: it could be an option, but a native solution in QGIS would be
preferred

So in summary: there are several options around for cleaning overlaps - but
it isn't as straight-forward and user friendly yet as it could/should be.

I haven't looked at the gaps yet ...

Thank you all and greetings,
Andreas


On Tue, 25 Jul 2023 at 13:13, Bernhard Ströbl via QGIS-User <
qgis-user at lists.osgeo.org> wrote:

> Hi Andreas,
>
> the Algorithm "Eliminate selected polygons" was originally created to
> address these questions. I usually imported the data into GRASS ran a clean
> there and reexported the results into a non topolgical dataset. Thus
> overlapping areas and gaps (only if closed) are identified and can be
> eliminated using the algorithm named above (selecting only a certain
> area-circumference relation) by merging it with the adjacent polygon with
> the smallest/largest area or the longest common boundary.
> For your case you could have used this algorithm after the union to
> identify overlaps.
> As for an algorithm that does all this in one go: You would need something
> that detects holes between polygons without the help of a toplogical data
> format.
>
> regards
> Bernhard
>
> > Andreas Neumann via QGIS-User <qgis-user at lists.osgeo.org> hat am
> 25.07.2023 12:01 CEST geschrieben:
> >
> >
> > Hi,
> >
> > A friend of mine has a dirty input data set with lots of overlapping
> > geometries (within the same layer) and asked me if there is a tool
> > within QGIS to automatically remove the overlaps and assign the
> > overlapping area to the neighbor polygon with the largest area.
> >
> > The solution was surprisingly hard to find, although I am pretty sure
> > there are multiple combinations of algorithms that would solve the
> > problem. Here is the solution I came up with:
> > https://github.com/qgis-ch/overlap_removal/tree/main [1]- perhaps you
> > have better ideas - more elegant solutions?
> >
> > Wouldn't it be great if QGIS had a processing tool to solve this overlap
> > cleaning within the same layer "out of the box" without having to use a
> > graphical model or a more or less complicated sequence of algorithms in
> > the processing toolbox? Apparently, ArcGIS has such a tool ...
> >
> > Saga and GRASS also might have such tools - but I couldn't get the SAGA
> > based QGIS plugin "Dissect and dissolve overlaps"
> > (https://plugins.qgis.org/plugins/dissect_dissolve_overlaps).
> >
> > The same problem exists for automatically filling small gaps in the
> > polygon data set ...
> >
> > Andreas
> >
> >
> >
> > Links:
> > ------
> > [1]
> https://github.com/qgis-ch/overlap_removal/tree/main_______________________________________________
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-- 

--
Andreas Neumann
QGIS.ORG board member (treasurer)
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