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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:

https://themes.jrc.ec.europa.eu/discussion/view/126358/inspire-download-services-published-providing-inspire-compliant-ps-data

https://themes.jrc.ec.europa.eu/discussion/view/72255/orientation-of-polygons-created-using-hale

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

At date, the ETF validator has a bug which leads to incorrect results for the counterclockwise orientation of polygons.

Please make reference to issue https://github.com/inspire-eu-validation/ets-repository/issues/60   and to the FAQ page in the ETF github repository (https://github.com/inspire-eu-validation/ets-repository ) 

You may find more details also in the last replies to the following TC discussion https://themes.jrc.ec.europa.eu/discussion/view/109106/fyi-interpreted-coordinate-order-flipped-in-gml-files-with-uri-format-srsname

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