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

[Alister Hood]

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>
> To: 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>
> 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
>
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