I have some polygons of a country, states, districts, communities and I want to simplify the geometry.

I already simplified the geometry of each layer but afterwards of course it doesnt fit with the geometry of the higher or lower administrative level.

I tried https://trac.osgeo.org/postgis/wiki/UsersWikiSimplifyPreserveTopology and mapshaper.org.

Any ideas?

  • Could you add some screenshots to show how your simplified layers do not fit the other administrative levels? The GRASS tool v.generalize is quite a powerful tool in simplifying whilst maintaining topology as asked in this post which might help.
    – Joseph
    Jan 7, 2016 at 14:59

4 Answers 4


The best approach I guess is using real topology data.

Then you can share the edges between data sets and simplify as much as you want.

Here is an example from the man behind the topology implimentation in PostGIS.



This can be achieved with the integrate tool in ArcGIS. Unfortunately, it is not available as-is in qgis/postgis (see this question) but a workaround exists with GRASS: See HERE !


Martin Davis has done the foundation work and the PostGIS team have now implemented this capability.

https://lin-ear-th-inking.blogspot.com/2023/03/simplifying-polygonal-coverages-with-jts.html and https://postgis.net/docs/ST_CoverageSimplify.html


Starting from QGIS 3.36 there are some algorithms on vector coverages


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