Type describing the thematic and geometric attributes and the associations of buildings. It is an abstract type, only its subclasses Building and BuildingPart can be instantiated. An _AbstractBuilding may consist of BuildingParts, which are again _AbstractBuildings by inheritance. Thus an aggregation hierarchy between _AbstractBuildings of arbitrary depth may be specified. In such an hierarchy, top elements are Buildings, while all other elements are BuildingParts. Each element of such a hierarchy may have all attributes and geometries of _AbstractBuildings. It must, however, be assured than no inconsistencies occur (for example, if the geometry of a Building does not correspond to the geometries of its parts, or if the roof type of a Building is saddle roof, while its parts have an hip roof). As subclass of _CityObject, an _AbstractBuilding inherits all attributes and relations, in particular an id, names, external references, and generalization relations. The name will be represented by gml:name (inherited from _GML) . list order for storeyHeightsAboveground: first floor, second floor,... list order for storeyHeightsBelowground: first floor below ground, second floor below ground,... The lodXMultiSurface must be used, if the geometry of a building is just a collection of surfaces bounding a solid, but not a topologically clean solid boundary necessary for GML3 solid boundaries. Intended function of a building. The values of this type are defined in the XML file BuildingFunctionType.xml, according to the dictionary concept of GML3. The values may be adopted from ALKIS, the german standard for cadastre modelling. If the cadastre models from other countries differ in the building functions, these values may be compiled in another codelist to be used with CityGML. Roof Types. The values of this type are defined in the XML file RoofTypeType.xml, according to the dictionary concept of GML3. Denotes the relation of an _AbstractBuilding to its building parts. The BuildingPartPropertyType element must either carry a reference to a BuildingPart object or contain a BuildingPart object inline, but neither both nor none. A BuildingInstallation is a part of a Building which has not the significance of a BuildingPart. Examples are stairs, antennas, balconies or small roofs. As subclass of _CityObject, a BuildingInstallation inherits all attributes and relations, in particular an id, names, external references, and generalization relations. Function of a building installation. The values of this type are defined in the XML file BuildingInstallationFunctionType.xml, according to the dictionary concept of GML3. Denotes the relation of an _AbstractBuilding to its building installations. The BuildingInstallationPropertyType element must either carry a reference to a BuildingInstallation object or contain a BuildingInstallation object inline, but neither both nor none. A BoundarySurface is a thematic object which classifies surfaces bounding a building or a room. The geometry of a BoundarySurface is given by MultiSurfaces. As it is a subclass of _CityObject, it inherits all atributes and relations, in particular the external references, and the generalization relations. Denotes the relation of an _AbstractBuilding to its bounding thematic surfaces (walls, roofs, ..). The BoundarySurfacePropertyType element must either carry a reference to a _BoundarySurface object or contain a _BoundarySurface object inline, but neither both nor none. There is no differentiation between interior surfaces bounding rooms and outer ones bounding buildings (one reason is, that ClosureSurfaces belong to both types). It has to be made sure by additional integrity constraints that, e.g. an _AbstractBuilding is not related to CeilingSurfaces or a room not to RoofSurfaces.