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GML explicitly implements * (see Note 1 below) the semantics of ISO 19107. The  The startOfArc  and endOfArc are defined in terms of bearings. The definition of “bearing” states that

Bearing is a data type used to represent direction in the coordinate reference system. In a 2D coordinate reference system, this can be accomplished using a “angle measured from true north” or a 2D vector point in that direction.

There are two variants mentioned in ISO 19107 for 19107 for expressing bearings: angle and direction.The semantics for angle is

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Although it may not be obvious, this definition matches the needs of the AI domain, as angles are usually expressed in degrees measured clockwise from the True North. The diagrams below explain why the “counter clockwise” convention stated in the ISO 19107 standard19107 standard, when combined with left-handed geodetic CRS (see also page Right-handed vs. Left-handed CRS) actually corresponds to a clockwise rotation in the AI domain.

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The symbol “deg” shall be used for the uom attribute of gml:startAngle and gml:endAngle elements.

Footnotes

* 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 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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