I want to merge some polygons into one using the merge tool. But when doing so, one of the polygons just disappears (see screenshot 2). Why does this happen? When I use the simplify tool before, it does not happen.

My system: QGIS 2.6.0; Linux Mint 17

merge tool

polygon right bottom disappeared

  • Does it disappear at all scales when zooming in and out? – SaultDon Nov 28 '14 at 19:25
  • Yes, it does appear also in different zooming – Antoine Nov 28 '14 at 19:56
  • I would run a geometry check on that shape (or the entire file) to start with and make sure it's valid with no errors. – Chris W Nov 28 '14 at 20:33
  • 2
    Yes vektor>geometry tools>check geometry did it! Two polygons had crossings within themselves. – Antoine Nov 28 '14 at 21:09

When specific shapes exhibit strange behavior as a result of a geoprocessing tool or editing operation, one of the first things to check is whether they have valid geometries. Geometry errors can result in null results or other errors. QGIS has a Check Geometry Validity tool, and another option would be to load it into GRASS and run v.clean. There are several tools available in other software as well.

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