Perimeter Encoding Direction

The GML Surface implements ISO 19107 GM_Surface whose exterior boundary shall be encoded counter-clockwise and any interior boundary encoded clockwise.

For aeronautical data, this implies that the outside perimeter and any eventual holes shall be encoded as shown in the figure below. This rule might be difficult to apply strictly on short term as existing aeronautical systems require data to be encoded in clockwise direction. For example, the Operating Procedures for Aeronautical Dynamic Data (OPADD) , indicates  that “points defining lateral limits of an area should be enumerated in clockwise order”.

In addition, in the current AIXM version 5, there is no need for encoding Surface holes. Eventual Airspace “holes” are encoded as subtractions of an AirspaceVolume, as explained in the “Airspace aggregation” section. Therefore, internal patches (rings) are excluded from the aviation data profile of GML.

Compliance with this ISO 19107  rule shall remain an objective for AIXM implementations. However, on short term it might be expected that certain AIXM data will not comply with this rule and application developers shall be aware of this exception. As the use of GML internal patches (rings) is not supported by this profile, non-compliance with this rule is not expected to have practical consequences. Existing GIS software often is rather lenient when it comes to this rule and often ignores it.

References