OrientableSurface consists of a surface and an orientation. If the orientation is "+", then the OrientableSurface is identical to the baseSurface. If the orientation is "-", then the OrientableSurface is a reference to a gml:AbstractSurface with an up-normal that reverses the direction for this OrientableSurface, the sense of "the top of the surface".
The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
<element name="OrientableSurface" type="gml:OrientableSurfaceType" substitutionGroup="gml:AbstractSurface"><annotation><documentation>OrientableSurface consists of a surface and an orientation. If the orientation is "+", then the OrientableSurface is identical to the baseSurface. If the orientation is "-", then the OrientableSurface is a reference to a gml:AbstractSurface with an up-normal that reverses the direction for this OrientableSurface, the sense of "the top of the surface".</documentation></annotation></element>