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I have a shapefile of buffered points in QGIS. I need to display all buffer polygons in this layer which overlap. I have experimented with the intersect tool, but this only seems to work if I am looking at the intersection of 2 separate layers otherwise it just says that all polygons intersect.

Does anyone have a QGIS solution?

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3 Answers 3

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Enable Topology Checker Plugin in Plugin Manager. Add your polygonal layer in Topology Rule Settings window, select "must not overlap" rule and add them. To see overlap errors click on Validate button.

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    This works however, it doesn't seem to detect exact overlaps i.e. polygons that are exactly the same. I need to select these as well
    – user1658170
    Commented Oct 24, 2014 at 14:33
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    Try convert polygons to lines (Vector - Geometry tools - Polygons to lines) and then create point layer from line intersections - Vector - Analysis Tools - Line Intersections. You receive points in intersections where line of one polygon feature intersect anotheк polygon feature. Then select overlaping polygon create spatial query using point and polygonal layers and you receive selected polygonal features which have overlaps.
    – spatialhast
    Commented Oct 24, 2014 at 14:53
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    Thanks for the suggestion. Ultimately what worked was adding 2 different rules to the Topology Checker. Rule 1 was defined as "must not overlap" to capture the overlaps and Rule 2 was defined as "must not have duplicates" to capture the exact overlaps.
    – user1658170
    Commented Oct 24, 2014 at 15:13
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    One additional question related to this question. Is there a way to select the errors in the original layer?
    – user1658170
    Commented Oct 25, 2014 at 13:17
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You can:

  • Field calculate an integer field named tempid using the expression @row_number

  • Union the layer, with field calculator output as Input Layer, no overlay layer

  • Aggregate with group by expression $geometry and aggregates Source expression = tempid, Aggregate function = count and Name = overlap_count

This will create a polygon layer with the column overlap_count which is the number of overlapping polygons:

enter image description here

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I had similar challenge but got some satisfactory workaround. Duplicate the same layer and select the spatial query tool. Add the original and the copied layers. Select overlap instead of intersection. Run it, and you find the intersecting buffers highlighted. You may proceed from there.

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    If it is a duplicate wouldn't all polygons overlap between the two layers?
    – HeikkiVesanto
    Commented Oct 17, 2017 at 13:32

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