I made some models in QGIS, mostly with the algorithms Union, Aggregate and Field Calculator while also using Fix geometries en Multipart to Singlepart at the start.

My output layer contains 7099 features but while executing the model, I got the following error message 4 times:

Feature could not be written to Union_0f2336d4_3f33_4bb1_8f67_87aefc7154d6: Could not add feature with geometry type MultiLineStringZ to layer of type MultiPolygonZ

It seems it has something to do with the Union algorithm. The input layers all consist of polygons. These are created in a farming GPS system.

Does anyone know how to solve this issue?

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    – Ian Turton
    Mar 15, 2022 at 11:31
  • are all of your geometries valid?
    – Ian Turton
    Mar 15, 2022 at 11:31
  • I use fix geometries as the first step so they should be valid from there right?
    – Rik
    Mar 15, 2022 at 12:04
  • Not necessarily, a quick isvalid check is probably a good idea
    – Ian Turton
    Mar 15, 2022 at 12:36
  • Ah okay, I didn't konw that. I don't get any further though, also trying v.clean. So I'll stick to what I had. Thanks for your help!
    – Rik
    Mar 15, 2022 at 13:15

1 Answer 1


I think I finally found the solution. The errors disappeared since the model starts with creating a spatial index and since I removed the step to reproject the layer.

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