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I have a polygon Shapefile, which I need to generalize. How can I generalize the poligons and ensure that there are no topological errors like "slivers".

I already tried:

  1. Vector -> Geometry Tools -> polygones to lines
  2. Vector -> Geometry Tools -> simplfy geometries ... here I can already see on the screen, that I am producing overlaps and gaps nevertheless now I try
  3. Vector -> Geometry Tools -> lines to polgones

and here the program show an error and crashes

enter image description here

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Are there any news on this issue? –  Oystein Nov 28 '13 at 9:11
unfortunately: no –  Kurt Nov 30 '13 at 19:39

4 Answers 4

GRASS is topology-aware. You can use v.generalize from the Processing toolbox to simplify polygons and if the input data is topologically correct so will the output.

enter image description here

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I just did a quick test, and the "Simplify Geometries' tool under the Vector menu appears to respect the topology. I tried with and without the "Enable topological editing" checked off under Snapping Options. Have you tried this?

EDIT: This was done directly on the polygon file--why would you convert to lines first?

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Hi Darren, thanks for the hint I will try it with enablr topological editing –  Kurt Feb 24 '12 at 14:33
Hi Darren, imho it does not respect topology. there is no difference with "enable topological editing". I supppose, this is because generalzing generates a new layer, while "enable topological editing" seems to work only with -selected - existing layers. Could you please do very very strong gerneralization and check again, if you have a correct topology now? thanks –  Kurt Feb 24 '12 at 14:47
Kurt; Interesting. Looks like you're right when you do a strong generalization! Sorry! –  Darren Cope Feb 24 '12 at 19:01
This checkbox is gone in QGIS 2.2 (perhaps earlier) in case anyone will search. –  lynxlynxlynx Mar 22 at 13:31

There is the same discussion here, my final answer being that SAGA does the job for you.

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Following your answer in that discussion, it seems SAGA does not preserve topology. At least not anymore. –  Oystein Nov 28 '13 at 9:12

Have you tried using MapShaper? I've found this to be a good way of generalizing polygons; there's also an offline version if you want to download the code.

An alternative could be convert this to GeoJson, and then TopoJson which supports "topology-preserving shape simplification" https://github.com/mbostock/topojson

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