Type describing the "root" element of any city model file. It is a collection whose members are restricted to be features of a city model. All features are included as cityObjectMember. Type describing the abstract superclass of most CityGML features. Its purpose is to provide a creation and a termination date as well as a reference to corresponding objects in other information systems. A generalization relation may be used to relate features, which represent the same real-world object in different Levels-of-Detail, i.e. a feature and its generalized counterpart(s). The direction of this relation is from the feature to the corresponding generalized feature. Type describing the abstract superclass for buildings, facilities, etc. Future extensions of CityGML like bridges and tunnels would be modelled as subclasses of _Site. As subclass of _CityObject, a _Site inherits all attributes and relations, in particular an id, names, external references, and generalization relations. Type describing the reference to an corresponding object in an other information system, for example in the german cadastre ALKIS, the german topographic information system or ATKIS, or the OS MasterMap. The reference consists of the name of the external information system, represented by an URI, and the reference of the external object, given either by a string or by an URI. If the informationSystem element is missing in the ExternalReference, the ExternalObjectReference must be an URI, which contains an indication of the informationSystem. Denotes the relation of an _CityObject to its addresses. The AddressPropertyType element must either carry a reference to an Address object or contain an Address object inline, but neither both nor none. Type for addresses. It references the xAL address standard issued by the OASIS consortium. Please note, that addresses are modelled as GML features. Every address can be assigned zero or more 2D or 3D point geometries (one gml:MultiPoint geometry) locating the entrance(s). Denotes the relation of an Address feature to the xAL address element.