diff --git a/GIS-data-export.md b/GIS-data-export.md
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@@ -40,6 +40,9 @@ Choose the saved .geojson file. It should be set up correctly as well, but doesn
 
 There is additional cell information such as population, height (for a heightmap), but also states, provinces, culture, etc. exported, all of which can be used in QGIS to render this information.
 
+Unfortunately, there are sometimes gaps or overlaps in the cell export data. You can find them in QGIS with _Vector -> Geometry Tools... -> Check Validity_. They are easy to fix by hand, if you need to (only needed if you want to do further processing with the data).
+
+
 ## Burg Data
 Burg data can be downloaded as _.csv_ file using the button in the _Burgs Editor_ . The downloaded file contains position information (longitude, latitude and height) for burgs. These can be imported into QGIS by choosing _Layer_ -> _Add Layer..._ -> _Add Delimited Text Layer..._