[GIS] Solving topological errors like invalid geometries in QGIS

invalid-dataqgistopology

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.

Best Answer

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/

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