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I have two layers to make an intersection. I use QGIS 2.18.22 and Vector - Geoprocessing Tools - Intersect - run is not working, because "Input layer A contains invalid geometries. Unable to complete intersection algorithm." Anyone having any idea how to fix it? Layers are here: https://uloz.to/tam/_BGkW3f7iUrDm

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    You need to fix the invalid geometry, there are lots of question on this site dealing with this problematic have a look and update your question if you're still stuck.
    – J.R
    Jul 25, 2018 at 9:33

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Upgrade your version to QGIS 3.2.

Then, in the toolbox (Menu processing -> Toolbox) you will find a tool called "fix geometries". Use this tool bevor making the intersection

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    Used QGIS 3.2, Menu Processing, Toolbox, Fix geometries works. However, Vector, Geoprocessing Tools, Intersection doesn´t work then. The note is just: "GEOS geoprocessing error: intersection failed."
    – Stefan
    Jul 26, 2018 at 10:10
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    May be this 3 points: 1. You have to use the intersection geoprocess with the corrected layer. 2. All your layers must have the same CRS (coordinate reference system) 3. Fix first all the layers you will use
    – Christophe P.
    Jul 26, 2018 at 10:45
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    Not these 3 problems in my case. When working with just a few polygons, it works. The problem is for the layers with many polygons which I had merged together before. After a few minutes it tells: "GEOS geoprocessing error: intersection failed." However, when first using Vector, Geometry Tolls, Multipart to Singleparts, the Intersection feature works.
    – Stefan
    Jul 26, 2018 at 12:22
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In QGIS 2.18 you can use GRASS v.clean from the processing toolbox to fix invalid geometry.

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