Splitting polgons in QGIS produces topology errors

Question

I am trying to split existing polygons but when I do, gaps and overlaps are created between the polygon I am cutting and adjacent polygons.

I have snapping switched on and have enabled topological editing. I also get lines appearing across the screen which then disappear if I zoom in or out.

For example in the image below the line with the red arrow pointing to it is not a polygon boundary and disappears when I zoom. Also if I delete a polygon I am sometimes left with strange dangling nodes that should not be there.

Answer 1

I had the same issue. You need to make sure that the polygon is free of any issues, like duplicated point or intersections.

Do this by :

1. Select your feature
2. Go Vector -> Geometry Tools -> Check Geometry Validity
3. Here choose Use only selected features to make the process faster
4. Hit OK and delete error if you have some
5. Now retry the splitting

Answer 2

Even after confirming the polygon layer's validity I still found that the Split Features tool left topology errors. In my experience, ensuring that Enable Topological Editing is turned on on the Snapping toolbar prevents this from happening.

Answer 3

You could try deleting the relevant polygons from the "Achlain_Current" shapefile and then running a Merge or Union tool with the "Restock" shapefile.

Another option is to try v.clean again on your new shapefile after you copied the polygons and run various tools such as deleting duplicate polygons, removing dangles etc.

Hope this helps!