gml:ImageDatum defines the origin of an image coordinate reference system, and is used in a local context only. For an image datum, the anchor definition is usually either the centre of the image or the corner of the image. For more information, see ISO 19111 B.3.5.
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.
Source
<element name="ImageDatum" type="gml:ImageDatumType" substitutionGroup="gml:AbstractDatum"><annotation><documentation>gml:ImageDatum defines the origin of an image coordinate reference system, and is used in a local context only. For an image datum, the anchor definition is usually either the centre of the image or the corner of the image. For more information, see ISO 19111 B.3.5.</documentation></annotation></element>