I have a huge amount of lines indicating boundaries of certains areas but they are most of time "opened" sometimes they do not have common vertices and sometimes are overlapping. I want to create a polygon by these lines that split the "BIG AREA POLYGON" into smaller polygon following the lines. How to manage this? I've tried the poligonize tool and the lines to polygons but results are not as expected.



result under polygonize tool polygonize tool result

result after lines to polygons tool lines to polygons tool result

  • Lines that have gaps in them won't build polygons for the gap area, or split polygons they cross. If the gaps are under a small size, a tolerance say of 1m, you could run a v.clean using the GRASS tool before running polygonize. You probaby aren't familiar with the GRASS tools so you might first try the snapping tools. But if the gaps are larger than an acceptable tolerance, as it appears from your image, you are going to have to fill in the gaps with new lines or snapping ends. You might get some of the work done by using the extend lines tool but manual work is likely needed as well.
    – John
    Jan 18, 2023 at 19:02
  • 1
    What do you mean by "results are not as expected."?
    – BERA
    Jan 19, 2023 at 6:15
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    I added some screenshots as "results not as expected" :)
    – Eppesuiga
    Jan 20, 2023 at 11:54
  • Maybe there are gaps in your line layer. Try snapping the line layer to itself then convert to polygons
    – BERA
    Jan 20, 2023 at 12:01
  • snapping manually?
    – Eppesuiga
    Jan 20, 2023 at 12:03

1 Answer 1


What you're looking at here are significant topology errors. Here's a good article on ways to fix them in QGIS. Once you've fixed the topology errors, the polygonizing tool will work as expected, assuming the topology fixes have created the correct boundaries for the polygons.

I know this isn't a direct answer to your question but since this has been unanswered for a while now, I thought pointing you into a direction might help.

  • thanks for your suggestion but I can't find v.clean.advanced in 'processing toolbox'
    – Eppesuiga
    Jan 31, 2023 at 9:30
  • I believe that means you don't have GRASS GIS installed. You can install it from the manage plugins menu. Here's a guide.
    – enor
    Jan 31, 2023 at 10:03
  • No, no, I have grass installed and trying to search 'v.clean' I will catch only v.clean and not the advanced tool
    – Eppesuiga
    Jan 31, 2023 at 12:05

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