How to check Topology in QGIS?

Question

I have two Shapefiles with Polygons ( City and Forerst)

Is there a Plugin who is able check the topology (QGIS)? Like "Knots do not overlap" or something like that? I dont want that they overlap.

Does anybody know a Plugin? Would be great!

Answer 1

You can use the GRASS Toolbox for that.

Cleaning of topology of a SHAPE file using the GRASS Toolbox

1. Load the SHAPE file into QGIS
2. Use existing GRASS mapset (or create a new one) with matching projection settings
3. Now you have to transfer the SHAPE file from QGIS to GRASS using Toolbox -> File management -> Import into GRASS -> Import vector into GRASS -> Import vector into GRASS from QGIS view (v.in.ogr.qgis). Some topological problems may be reported depending on the quality of your SHAPE file.
4. These topological problems we can fix now in GRASS via toolbox using the topology tools: Vector -> Develop map -> Toolset for cleaning topology of vector map -> v.clean.break. Define a threshold in map units (for example 0.5 [m] or likewise). Analyse the printed report, probably the threshold needs to be adjusted or a different topology cleanup tool to be used. Click then on view output to show the result as map.
5. Export the cleaned map to SHAPE format: File management -> Export from GRASS -> Export vector from GRASS -> v.out.ogr

Answer 2

The new Topology Checker Plugin will be available in the next release. You can see it at work in this video: http://www.youtube.com/watch?v=huhkTZkoKC8

More info: https://github.com/qgis/Quantum-GIS/pull/356