I'm digitising a large number of maps from hand drawn mud maps and digital images, and although I have used the snapping tools ("snap to vertex and segment", Tolerance 10 pixels, avoid intersections) I consistently get ~30 geometry errors. My workflow is as follows:
- digitise (polygons) using snapping settings as above
- Use GRASS tools to remove most bad geometry (v.in.ogr.qgis, snapping threshold 0.02 min size 0.0001)
- Dissolve polygons with same attribute
- Check geometry validity
I consistently get around 30 errors that persist, but if I tweak the GRASS setting anymore it changes the map into an unrecognisable blob. Tutorials like this one tell you how to find the errors and then it say "fix them", but I guess I haven't figured out a good way to fix them? The errors are always of this type, a green cross where polygons meet and their handles overlap:
It seems the only way I can figure out to fix them is to nudge one polygon slightly to the side, so it's no longer overlapping that one point anymore, but I worry that will mess with any spatial analyses I run?
My questions are:
- How do I avoid getting all these errors?
- How do I fix them if they do occur?
- Are they important?
I'm not a QGIS power user.
This is what the problem looks like when I nudge the polygons apart, the top three polygons all join in the same node, while the fourth (bottom right) is the one that is overlapping and causing the problem.
zoomed out (area ~1x1m), error highlighted with red cross