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my problem: I have a polygon-file in shape-format, which I need to generalize. How can "split" the polygons into lines, generalize and than "rebuild" the polygons, so that I can ensure, that there are no topological errors like "slivers". Please give a rookie some adive howto do this in quantumgis.

Thank you very much


I already tried:

Vector -> Geometry Tools -> polygones to lines

Vector -> Geometry Tools -> simplfy geometries

here I can already see on the screen, that I am producing overlaps and gaps nevertheless now I try

Vector -> Geometry Tools -> lines to polgones

and here the program show an error and crahes

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

2 Answers 2

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

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

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