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I use QGIS, version 2.18.9. I´ve several polygon-shapes with topological errors. Before I tried to apply the "intersection"-tool and got the message: Input layer A contains invalid geometries (feature 77). Unable to complete intersection algorithm. Firstly, I used the "check-validity"-tool, which detected some line-intersection and double nodes.

Then, I tried the "topology checker" with "must not have invalid geometries".

I´ve had the impression, that all errors where found then. So I applied the LWGEOM algorithms "Make valid" but got a error another messages ([error 87] falscher Parameter see log for more details).

I´m an absolute QGIS-beginner. So, I have not a single idea how to solve this problem. It has to be a simple automatically way, ´cause I´ve a big dataset here.

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The most successful way I've ever had of correcting topological errors in QGIS is actually to use GRASS. You can either:

  1. run v.clean from the processing toolbox (manual here https://grass.osgeo.org/grass73/manuals/v.clean.html);
  2. use the 'Repair Geometry' plugin (which is an automation of the v.clean tool); or if all else fails
  3. import the dataset into GRASS proper and then export it out again as whatever format you want

I've never had number 3 fail for me and there is a good walkthrough of how to do it here: https://faunaliagis.wordpress.com/2013/08/14/bad-bad-polygon-fixing-invalid-geometries-with-quantum-gis/

  • Thank you Ed. Yesterday I tried the intersection tool with an older version (2.18.0) an got at least with two layers no other problems. Maybe it is the version which causes the problems. But nevertheless I had the same problems with another layer. I guess, I´ll try your suggestions. V. clean I already applied but didn´t understand... – Sophie B. Aug 1 '17 at 8:08
  • V.clean is a bit daunting as it gives you so many options to try. I'd recommend the Repair Geometry plugin to streamline it a little bit. – Ed Rollason Aug 1 '17 at 10:07
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LWGEOM plugin is good option to fix geometry, works for my problems (like "invalid geometry") well. @Sophie B., your [error 87] might be solved by correct path definition to LWGEOM binaries. You need to locate it and set it in Processing > Options >> Providers >> LWGEOM algorithms, see screenshot below (QGIS v2.18.15).

Processing > Options >> Providers >> LWGEOM algorithms

Then you can validate the results with Vector > Geometry Tools > Check Validity, or by Topology Checker plugin.

  • Careful just for this, seen on the git : "Plugin DOESN'T work with postgis 2.0.* installations" – gisnside Dec 21 '17 at 14:51
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Another option is to use Geometry Checker plugin. This tool let you also to check geometry validity, properties, condition and of course topology. If you have a big dataset and return hundreds of errors. Try to fix it several times. Check out this post which describe how to clean topological error with Geometry checker and also v.clean tool. The result from using each tool is also compared. So you can get a better understanding in using the tools

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