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I have a polygon shapefile, which I need to generalize.

How can I generalize the polygons and ensure that there are no topological errors like slivers?

I have already tried:

  1. Vector -> Geometry Tools -> polygons 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 polygons

and here the program shows an error and crashes

enter image description here

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4 Answers 4

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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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+100

This can be seen as a preliminary to @Underdark's answer whereby you can clean the topology of the vector layer before generalizing. GRASS has a v.clean function which contains a number of tools to repair the layer such as:

  • snap which 'snaps' lines to the nearest vertex
  • rmdangle which removes any annoying dangles
  • rmdupl which removes duplicated geometry features

and various others shown in the image below:

v.clean

You can also set the threshold depending on how sensitive you want the tools to run. Personally, I tend to use the Check Geometry Validity feature in QGIS (Vector > Geometry Tools > Check Geometry Validity) to check for any errors and if any exist, use the appropriate tool in v.clean. Once the errors are removed, you should be in a better position to generalize.

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You could try 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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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.

This was done directly on the polygon file.

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