I'm using QGIS 2.2 and I have a polygon shapefile which was created by someone else. I have managed to get the shapefile to a topologically clean state by running v.clean but when I tried to cut the existing polygons to show a corridor (for a new powerline) gaps and overlaps then appear when I run topology. I have 'enable topological editing' crossed and snapping on and avoid intersection. Each time I try to cut the polygons new gaps and overlaps appear in different places.

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    Any time you cut a shapefile polygon you are adding vertices to it, and not to those it is coincident with; thus they are no longer topologically correct. To simplify a bit the clean process introduces vertices in the coincident polygons to match the one you cut, and thus create a correct topology. In ArcGIS after editing layers in a geodatabase topology you have to validate them, which does the same thing. I've always liked the topology/precision discussion here in its FAQ: ian-ko.com – johns Nov 4 '14 at 13:41
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    Are you getting gaps/overlaps when you cut a single polygon into three pieces (corridor/sides)? Because that would suggest something is wrong with the tolerance settings or coordinate system of the shapefile. How are you cutting - using another polygon/layer or manually drawing? How many layers are you working with - just the one? You've enabled topological editing, but is that layer(s?) participating in the topology? – Chris W Nov 4 '14 at 19:31
  • It does seem possible it could be something to do with the co-ordinate system and snapping tolerance levels as I found if I cut very zoomed in and did each little bit I got less errors than when zoomed out. Thank you for the link @johns I couldn't find a fix for it in QGIS, the tool in the article seems to only be for ArcGIS. – Jillian Nov 11 '14 at 16:47

Instead of cutting out polygons, an alternative is to reshape your polygon (Toggle editing > Edit > Reshape Features) into your desired corridor. The topology should remain intact as no polygons are being removed.

  • How would you use reshape to cut a corridor into a polygon? Reshape would allow you to alter the boundary, but I don't see how you could divide it in two (or three, depending on how you look at it). – Chris W Nov 4 '14 at 19:34
  • Thanks @ChrisW, probably should have asked the OP the same questions you asked in your comment above. I only mentioned the reshape if it was a possible alternative but you're right, you can't divide polygons with this function. – Joseph Nov 5 '14 at 10:07
  • Hi thanks for the suggestion but it didn't work. As mentioned it just changed the boundary but I still got the errors with it. I'm wondering if something has happened when I run v.clean. It's very strange as the shapefile will be topologically clean according to topology checker then when I split the polygons by snapping on a vertex at either side of the polygon gaps and overlaps then appear. If I split the polygon but do not snap onto any existing vertices then I get no error but this means my lines then look messy, especially where I want to connect to a corner. – Jillian Nov 11 '14 at 16:36
  • Sorry, I just noticed the first comments on my question. @Johns explanation seems to make sense as the gaps and overlaps appear to the adjacent polygons to the ones I am cutting. – Jillian Nov 11 '14 at 16:41
  • No worries. Apologies my solution didn't work, hopefully you will still get the results you want! – Joseph Nov 11 '14 at 16:46

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