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Short version: I have a layer with 19483 features with plenty of topological errors and I would like to fix/review them as many as possible in an automated way but don't know how to do it. Additionally, since I will have to repeat the same process with many other layers with a similar amount of features and errors, I would like to create a model/script to save time.

Long version: I have this layer (7MB) with 19483 features. After using topology checker plugin I find the following topological errors:

  • Gaps: 228 errors. Many of these gaps are the result of polygons with very small areas that should not exist.
  • Must not overlap: 106 overlaps. Again, most of the problems arise because there are edges which are very close one to another (0.2mm close) but they should be in the same place instead.

Additionally, if I enable Geometry Checker plugin I found 186 sliver polygons, 159 polygons with less than 1sqm. I am not sure whether I may have even more errors that I have not yet discovered.

I am pretty new to topology (in fact, it is the first time I have to deal with it), but from what I have read, I understand that there are three ways to deal with topology checks/fixes:

  • Automated checks but manual fixes using Topology checker plugin.
  • Automated checks semi-automated fixes using Geometry Checker plugin.
  • Automated checks and fixes using GRASS' v.clean

If I am not wrong, after reading v.clean's documentation, it looks promising and I may solve most of the errors by using the snap tool, as if I am not wrong, would snap/move vertexes that are very close when they should be snapped instead.

Another interesting feature is that I can integrate v.clean in a model, which will make my life much better since I will have to repeat the same process with many other layers with a similar amount of features and errors.

Unfortunately, I haven't succeeded in doing so. I created the following model but I might be doing something wrong since it creates more features instead of reducing its number. enter image description here

enter image description here

Did I do something wrong or are there better ways to deal with it?

  • Does your model complete? It's no surprise that you get more features after cleaning: any pair of intersecting polygons become three after topological cleaning, since the overlap becomes a new polygon. – Micha Aug 21 '18 at 7:44
  • Maybe I am not using the right tool, then. What I wanted to do is to simplify geometry first in terms of moving vertices only. What I expected is to have the same number of features or even less if by moving vertices of really small polygons (areas < 1sqm) disappear. Does that make sense? – ccamara Aug 21 '18 at 15:44
  • And about your first question: is my model not working on your side? – ccamara Aug 21 '18 at 15:45
  • If you want to simplify the polygon geometry, the v.generalize might be the tool you're looking for. – Micha Aug 22 '18 at 7:30
  • Thanks! Although I still don't understand the difference with vclean's snap, looks like what I was looking for. I will also try to remove small areas first, although I haven't succeeded yet with v.clean's rmarea – ccamara Aug 25 '18 at 5:01

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