Take the 2-minute tour ×
Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It's 100% free, no registration required.

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

share|improve this question

This question has an open bounty worth +100 reputation from underdark ending in 6 days.

This question has not received enough attention.

    
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

share|improve this answer

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?

share|improve this answer
    
Hi Darren, thanks for the hint I will try it with enablr topological editing –  Kurt Feb 24 '12 at 14:33
1  
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
1  
Kurt; Interesting. Looks like you're right when you do a strong generalization! Sorry! –  Darren Cope Feb 24 '12 at 19:01
1  
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.

share|improve this answer
    
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

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.