[QGIS-Developer] QGIS vs GEOS validity check

[Jeff McKenna]

Hi JF,

Regarding GEOS, please consider asking for more details through the 
project's vibrant forum (subscribe at 
https://lists.osgeo.org/mailman/listinfo/geos-devel ), I know there was 
a big release made yesterday and some chatter (project home: 
https://libgeos.org/ )  GEOS is one of those core libraries, who would 
greatly appreciate your direct feedback.

PS. hello from the east coast

Thanks,

-jeff



-- 
Jeff McKenna
GatewayGeo: Developers of MS4W, MapServer Consulting and Training
co-founder of FOSS4G
http://gatewaygeo.com/



On 2022-06-03 4:25 p.m., Bourdon, Jean-François (DIF) via QGIS-Developer 
wrote:
> Intranet logo
> 
> Hi Devs,
> 
> I had a surprising issue with a specific vector layer and a clip 
> operation. Validity check of the geometries seem to be done differently 
> if native:clip is used inside a processing script or manually through 
> the GUI as a vector layer containing a linestring with duplicate node 
> like LINESTRING ((0 0, 1 0, 1 0))  won't cause an issue when the clip is 
> done with the GUI but will halt if done inside a processing script (on 
> QGIS 3.22.6).
> 
> Also, if I run */qgis:checkvalidity/* with the *GEOS* method, no error 
> is found, but if I run it with the *QGIS* method, it finds one error 
> (/line 1 contains 2 duplicate nodes starting at vertex 2/). So here are 
> my questions: what exactly is checked when using the GEOS method and 
> what is checked using the QGIS method? Also, do all processing tools 
> (qgis: and native: ones) use the same kind of check?
> 
> I didn't find anywhere the details of what would be considered an error 
> according to the "QGIS spec". Is it documented somewhere or do I need to 
> look for hints inside the source code? As for the GEOS method, I only 
> found some error examples in the PostGIS documentation and in a FME page 
> (https://community.safe.com/s/article/invalid-ogc-geometry-examples 
> <https://community.safe.com/s/article/invalid-ogc-geometry-examples>). 
> Is there an official documentation of what geometries are invalid 
> regarding GEOS?
> 
> Thanks a lot for any pointer
> 
> *Jean-François Bourdon, ing.f.*
> 
> *Analyste en télédétection*
> *Direction des inventaires forestiers*
> *Ministère des Forêts, de la Faune et des Parcs*
> 5700, 4^e Avenue Ouest, local A-108
> Québec (Québec) G1H 6R1
> Téléphone : 418 627-8669, poste 704304
> *_jean-francois.bourdon at mffp.gouv.qc.ca_*
> *mffp.gouv.qc.ca* <http://www.mffp.gouv.qc.ca/accueil.jsp>
> 
>