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They are closed shape, i.e. the first latitude/longitude pair has to be equal with the last one.
As stated in section 10.5.11.1 of the GML Standard: “Every gml:curveMember references or contains one curve, i.e. any element which is substitutable for gml:AbstractCurve. In the context of a ring, the curves describe the boundary of the surface. The sequence of curves shall be contiguous and connected in a cycle”. In the special case that there is only one curve member in a gml:Ring, this means that the curve member itself needs to form a cycle, thus the need for the last position to be equal with the first one.
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| In GML 3.3 there are new compact encodings for geometry primitives, such as SimplePolygon that do not require the repeated last coordinate. However, these are not available yet in AIXM 5.1, which uses GML 3.2.1. |
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