The GML Surface implements ISO 19107 GM_Surface whose exterior boundary shall be encoded counter-clockwise and any interior boundary encoded clockwise.
(A link to a web site that provides a simple method for testing if a polygon is encoded clockwise or counter-clockwise: http://stackoverflow.com/questions/1165647/how-to-determine-if-a-list-of-polygon-points-are-in-clockwise-order.)
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 be considered a long term 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. Existing GIS software often is rather lenient when it comes to this rule and often ignores it. Therefore a gradual application of the ISO 19107 rules, staring for example with new data, is recommended.