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While running topology against two identical polygon (parcel) datasets in ArcGIS and QGIS, QGIS detects errors that ArcGIS does not. In four separate instances in my township-wide parcel dataset QGIS flags "invalid geometry" (screenshot 1) where two island polygons touch at a point within the larger containing polygon (screenshots 2 and 3). Notice that QGIS does not have an issue with a number of island polygons as long as they do not touch each other (adjacent parcel to the NE in screenshots 1 & 2).

Why is this an invalid geometry error?

screenshot 1

screenshot 2

screenshot 3

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    which version of QGIS, and are you using the standard geometry checker? Also, have you tried making the error column wider, in case you're only seeing part of the message?
    – Steven Kay
    Dec 21, 2016 at 19:15
  • QGIS 2.18.1 64-bit on Windows 10. I am using the Topology Checker -- the one available from the Vector pull-down menu. I am seeing the full message, I just cropped the screenshot. The message displays the error type ("invalid geometry"), layer name (very long PostGIS feature class), and the Feature ID. @StevenKay
    – Atanas Entchev
    Dec 21, 2016 at 19:27
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    ah, that plugin was disabled on my setup. You could try Vector > Geometry > Check Validity, that should output several layers with error features split out, and (hopefully) more descriptive messages.
    – Steven Kay
    Dec 21, 2016 at 19:44
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    I have found that making a very trivial buffer (0.0001 of a metre) into a new layer has fixed errors like this for me in the past. ciao
    – BillW
    Dec 22, 2016 at 3:00
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    Using QGIS 2.1821 I also faced the same problem, and ended using the combination Vector > Geometry > Check Validity suggested by @steven-kay and then I repaired the problems in the "Invalid Output" generated layer using the Grass v.buffer.distance with 0.00001m in both parameters (buffer distance... and maximum distance between...) as suggested by billw. Thanks
    – nanunga
    Jun 26, 2018 at 0:49

1 Answer 1

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This is a partial answer, summarizing the comments to my original question and my experience following the commenters' recommendations:

  • The "Topology Checker" and "Geometry -> Check Validity" tools generate different outputs
  • "Geometry -> Check Validity" generates three output layers with much more descriptive output regarding what might be wrong with the data
  • The particular issue I am experiencing might be related to bug #15377

Thanks to Steven Kay and eclipsed_by_the_moon.

ADDED on 2016-12-23: Editing the data to make the island polygons not touch (see screenshot) makes the "invalid geometry" error go away. In other words, QGIS considered my initial example a self-intersecting polygon, and therefore an invalid geometry (an error), even though this is a perfectly legitimate configuration in parcel fabric mapping.

For the record, as mentioned in the original question, ArcGIS does not flag this instance as an error. I had tagged my question with "arcgis" in addition to "qgis" in order to make that point, but a moderator removed the "arcgis" tag.

"invalid geometry" error resolved

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  • Is it possible to split the polygon using the split features tool? Maybe the 'Check geometries' plugin can help to find a geometry error.
    – eclipsed_by_the_moon
    Dec 21, 2016 at 21:08
  • QGIS does not allow me to split the feature because "the geometry is invalid", @eclipsed_by_the_moon . I'll keep investigating. The strange part is that ArcGIS does not detect any errors on an identical dataset...
    – Atanas Entchev
    Dec 21, 2016 at 21:21

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