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See ISO/DIS 19136 Clause 10. Any geometry element that inherits the semantics of AbstractGeometryType may be viewed as a set of direct positions. All of the classes derived from AbstractGeometryType inherit an optional association to a coordinate reference system. All direct positions shall directly or indirectly be associated with a coordinate reference system. When geometry elements are aggregated in another geometry element (such as a MultiGeometry or GeometricComplex), which already has a coordinate reference system specified, then these elements are assumed to be in that same coordinate reference system unless otherwise specified. The geometry model distinguishes geometric primitives, aggregates and complexes. Geometric primitives, i.e. instances of a subtype of AbstractGeometricPrimitiveType, will be open, that is, they will not contain their boundary points; curves will not contain their end points, surfaces will not contain their boundary curves, and solids will not contain their bounding surfaces.
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