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I have two QGIS layers I created from shape files. I wanted to find which regions from one layer intersect with districts from the other layer, and the intersect tool does that perfectly. I find however that in part of my map, the regions and districts coincide, but due to either rounding or tiny errors in my data, or something, there are extra intersections. For example a region along the border of one district appears to (incorrectly) intrude a tiny bit into the next district.

I am looking for a way to create a list of intersections, but not count intersections where the area of the intersection is less than some tiny amount. (I would be okay expressing the tolerance in any units, either as an area of as a distance)

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In the select by location tool have you tried using the 'overlaps' geometric predicate instead of 'intersects'? Intersects will select polygons who have touching, but not overlapping boundaries, whilst overlaps must have one coordinate within the boundary of the other. This tool also has a precision parameter that you can tweak to get the desired result if there are some topological errors in your data.

Here is a discussion on intersects vs overlaps What is the difference between Intersect & Overlap in ArcGIS Server?

  • I have never used ArcGIS (I have been doing all of my work in QGIS). Maybe I should look into switching... – Morris Nov 16 '17 at 3:06
  • The select by location tool is in QGIS. The discussion simply highlights the difference between intersect and overlap – Liam G Nov 16 '17 at 3:35
  • I see. Thank you. Do you know how to figure out what units Precision is in? All of my layers are in EPSG 4269. – Morris Nov 16 '17 at 4:22
  • Should be your map units i believe – Liam G Nov 16 '17 at 4:26

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