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Orientation of polygons counter-clockwise: Revision

GML encoding asks for orientation of the surface boundary to be consistent with the upward normal i.e. exterior boundary counter-clockwise and interior boundaries clockwise.

This because GML 3.2.1 standard directly refers the ISO 19107  in which we read ( ISO 19107:2003, cl. 6.3.17: GM_Surface) " the orientation of the surface chooses an "up" direction through the choice of the upward normal, which, if the surface is not a cycle, is the side of the surface from which the exterior boundary appears counterclockwise"

Most transformation software tools (e.g. HALE and FME) allow for producing counter-clockwise polygons when creating INSPIRE datasets.

You may find more details on 'how-to' in the following TC discussions:

Outer ring of polygon not being clockwise is one of the most common errors when validating INSPIRE GML datasets.

Discussions are ongoing also on the github repository for the ETF validator

you may find more details also in the last replies to the following TC discussion

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