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I have a shapefile layer with >27,000 polygons. I want to dissolve these together to create a footprint layer. The Dissolve tool works fairly well, but it has filled in what should be rings in the new layer. (Same result in both QGIS 2.14 and 3.4). They are gaps in the original layer, surrounded by multiple polygons, (i.e. these are true spaces, not rings in a single polygon, and they should not be filled). Can anyone suggest a way to avoid this happening, please? Or a workaround to fix the layer now that the rings have been filled?

Starting layer; enter image description here

Post Dissolve tool - with rings erroneously filled enter image description here

Desired output overlaid on the first dissolved layer showing the filled rings. enter image description here

Edit

I have now managed to get it to work, so perhaps it was a bug or an issue with my input layers? Would still like to know why it was happening to avoid a repeat in the future.

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  • For clarity, please Edit the question to contain a screenshots of your before, after, and desired outputs. It would also help to specify the data format of the layer source.
    – Vince
    Jul 24, 2019 at 12:41
  • What was your solution as I am having the same issue (QGIS 3.4.7 Maderia)
    – kungphil
    Nov 7, 2019 at 13:35
  • @kungphil, sorry I don't have a definite answer. I think it might have been an issue with having multipart polygons in my original layer? You might try ensuring all of your polygons are definitely single part before trying the dissolve.
    – EcologyTom
    Nov 8, 2019 at 14:09

1 Answer 1

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Just in case any one else finds this question, I think the problems were occurring because of geometry errors. Unfortunately, these were not picked up with the QGIS check and fix geometry tools. However, I was able to run the layer through ArcGIS repair geometry before using dissolve, and this seemed to resolve the issue. I've had no problems with it since.

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