[Qgis-user] Is this a bug in the vector clipping algorithm?

[Richard McDonnell]

Hi,
Just a quick one, you could try the Topology Checker

https://docs.qgis.org/2.8/en/docs/user_manual/plugins/plugins_topology_checker.html

This should help you locate things like Self Intersections. You will have to manually fix the error, but at least you have a way to find the issues.
Regard’s,

Richard.



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From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Alister Hood
Sent: 01 June 2021 23:06
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Is this a bug in the vector clipping algorithm?

OK, I thought I had tried "Check validity" with both QGIS and GEOS methods, but I must have got confused and only used GEOS method, which doesn't detect an error.
The first tool I tried was actually "Fix geometries", which also thinks the geometries are valid.  Maybe fix geometries uses GEOS and needs to be enhanced with a QGIS method.
It seems strange for other algorithms like "clip" to do more fixing than "fix geometries", especially in a case like this where you would think the feature could be clipped without any "fixing".

Thanks.

Date: Tue, 1 Jun 2021 02:45:51 -0700 (MST)
From: Andrea Giudiceandrea <andreaerdna at libero.it<mailto:andreaerdna at libero.it>>
To: qgis-user at lists.osgeo.org<mailto:qgis-user at lists.osgeo.org>
Subject: Re: [Qgis-user] Is this a bug in the vector clipping
        algorithm?
Message-ID: <1622540751842-0.post at n6.nabble.com<mailto:1622540751842-0.post at n6.nabble.com>>
Content-Type: text/plain; charset=us-ascii

Alister Hood-2 wrote
> I haven't been able to find a tool for validating or repairing geometries
> that thinks there is a problem with the original geometry

Hi Alister,
what tool have you used to check the geometry?

Have you tried to use the Geometry Checker core plugin (Vectror->Check
Geometries) or the "Check validity" (qgis:checkvalidity) processing tool
(with the QGIS method)?
They both report errors: "duplicate node", "self contact", "duplicate
nodes".

Also the ArcMap/ArcGIS Check Geometry tool reports "self intersections".

Anyway it seem the QGIS Clip tool does a "sanitize" of the intersection
result (always converting to Multi Type).

Regards.

Andrea Giudiceandrea

Please note that the existing 01-6476XXX / 046-9426XXX numbering ranges will no longer be available after 30th April 2021.
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