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* Note that this is true for classes of spatial primitives (e.g. GM_Arc) but in general not their operations. In the ISO 19107, startOfArc and endOfArc are operations, aka constructors which derive other values from the GM_Arc. However, GML/ISO 19136 defines the UML Class ArcByCenterPoint (Figure D.48) in a GML profile of ISO 19107 where startOfArc and endOfArc are attributes. Although not explicitly stated in the GML standard, we can infer that the ISO 19107 semantics for operations carry over to the identical semantics for the corresponding attributes with the same name in the GML profile of ISO 19107 (ISO 19136, Annex D).
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