A gml:AbstractDatum specifies the relationship of a coordinate system to theearth, thus creating a coordinate reference system. A datum uses a parameter or set ofparameters that determine the location of the origin of the coordinate reference system.Each datum subtype may be associated with only specific types of coordinate systems. Thisabstract complex type shall not be used, extended, or restricted, in a GML ApplicationSchema, to define a concrete subtype with a meaning equivalent to a concrete subtypespecified in this document.
Database handle for the object. It is of XML type ID, so is constrained to beunique in the XML document within which it occurs. An external identifier for the object inthe form of a URI may be constructed using standard XML and XPointer methods. This is doneby concatenating the URI for the document, a fragment separator, and the value of the idattribute.