gml:DirectedObservationAtDistance adds an additional distance property. This is the distance from the observer to the subject of the observation. Clearly this applies only to certain types of observations such as visual observations by people, or observations obtained from terrestrial cameras.
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="DirectedObservationAtDistance" type="gml:DirectedObservationAtDistanceType" substitutionGroup="gml:DirectedObservation"><annotation><documentation>gml:DirectedObservationAtDistance adds an additional distance property. This is the distance from the observer to the subject of the observation. Clearly this applies only to certain types of observations such as visual observations by people, or observations obtained from terrestrial cameras.</documentation></annotation></element>