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.
<complexType name="OrientableSurfaceType"> <complexContent> <extension base="gml:AbstractSurfaceType"> <sequence> <element ref="gml:baseSurface"/> </sequence> <attribute name="orientation" type="gml:SignType" default="+"/> </extension> </complexContent> </complexType>
<attribute name="orientation" type="gml:SignType" default="+"/>