General rule

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.

Dependence on the "up" direction

The direction of "clockwise" and "counter-clockwise" do not depend on the axis order of the given CRS. They just depend on where "up" is. In EPSG:4326, "up" is implied and pointing from the center of the earth outwards. When a surface is viewed from the side where "up" is, then – following ISO 19107 - the exterior boundary of the surface shall be encoded counter-clockwise while the interior shall be encoded clockwise.

Accordingly, the encoding of surface boundaries – counter-clockwise for exterior boundary, clockwise for any interior boundary - is the same in both left- and right-handed systems; see the following figure.

If arcs are part of the internal or external perimeter, the direction of these arcs needs to be in line with the overall orientation of the boundary. For arcs described by centre point and start/end angles, the encoding needs to take into consideration the convention for measuring arcs in left-handed and right-handed geodetic CRS, as detailed in the body of this document. The figure below shows some examples where arcs are part of a surface's boundary.

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