[Qgis-user] Point order in WKT export from polygon layer

[Andrea Giudiceandrea]

> [Qgis-user] Point order in WKT export from polygon layer
> Reetz, Michael (NLPVW) Michael.Reetz at nlpvw.niedersachsen.de
> Mon Aug 5 05:33:31 PDT 2024

> I've created a csv-file with coordinates in WKT-format by exporting a polygon layer with the "Save object as ..."-option ("Objekte speichern als ..." in the German QGIS version). I need this for querying data from GBIF via their API using JSON input. That has worked for the last years. But using the same JSON files this year gives an error because the point order is clockwise. I've found that according to the OGC standard the order should be counter clockwise for polygons and clockwise for excluded areas inside a polygon. It seems as if GBIF is following this standard now.

Hi Michael,
I suppose you are exporting the layer in the CSV format using the Layer 
option GEOMETRY = AS_WKT.

Instead, I suggest you to use the Field Calculator to add a real or 
virtual field for such layer and populate it with the WKT representation 
of the geometry [1]. You can force a particular winding of the geometry 
using either the force_polygon_ccw [2] or force_polygon_cw [3] function.

For example, using the following expression:

geom_to_wkt(  force_polygon_ccw( @geometry ) )

Best regards.

Andrea Giudiceandrea


[1] geom_to_wkt function 
https://docs.qgis.org/3.34/en/docs/user_manual/expressions/functions_list.html#geom-to-wkt
[1] 
https://docs.qgis.org/3.34/en/docs/user_manual/expressions/functions_list.html#force-polygon-ccw
[2] 
https://docs.qgis.org/3.34/en/docs/user_manual/expressions/functions_list.html#force-polygon-cw