3.1.2. Core model

The base class of all thematic classes within CityGML’s data model is the abstract class _CityObject. _CityObject provides a creation and a termination date for the management of histories of features as well as generic attributes and external references to corresponding objects in other data sets. _CityObject is a subclass of the GML class Feature, thus it may inherit multiple names from Feature, which may be optionally qualified by a codeSpace. This enables the differentiation between, for example, an official name from a popular name or names in different languages. The generalisation property generalizesTo of _CityObject may be used to relate features, which represent the same real-world object in different LoD, i.e. a feature and its generalized counterpart(s). The direction of this relation is from the feature to the corresponding generalised feature.

Features of _CityObject and its specialized subclasses may be aggregated to a CityModel, which is a feature collection with optional metadata. Generally, each feature has the attributes class, function, and usage, unless it is stated otherwise. The class attribute can occur only once, while the attributes usage and function can be used multiple times. The class attribute describes the classification of the objects, e.g. road, track, railway, or square. The attribute function contains the purpose of the object, like national highway or county road, while the attribute usage defines whether an object is e.g. navigable or usable for pedestrians. The attributes class, function and usage are specified as gml:CodeType. The values of these properties can be enumerated in code lists. Furthermore, for each feature the geographical extent can be defined using the Envelope element. Minimum and maximum coordinate values have to be assigned to opposite corners of the feature’s bounding box.

Fig. 3.1 Core Model and thematic top level classes

The subclasses of _CityObject comprise the different thematic fields of a city model, in the following covered by separate thematic models: building model (_AbstractBuilding), tunnel model (_AbstractTunnel), bridge model (_AbstractBridge), city furniture model (CiyFurniture), digital terrain model (ReliefFeature), land use model (LandUse), transportation model (TransportationObject), vegetation model (_VegetationObject), water bodies model (_WaterObject) and generic city object model (GenericCityObject). The latter one allows for the modelling of features, which are not explicitly covered by one of the other models. The separation into these models strongly correlates with CityGML’s extension modules, each defining a respective part of a virtual 3D city model.

3D objects are often derived from or have relations to objects in other databases or data sets. For example, a 3D building model may have been constructed from a two-dimensional footprint in a cadastre data set. The reference of a 3D object to its corresponding object in an external data set is essential, if an update must be propagated or if additional data is required (like the name and address of a building’s owner in a cadastral information system). In order to supply such information, each _CityObject may have External References to corresponding objects in external data sets. Such a reference denotes the external information system and the unique identifier of the object in this system.

CityObjectGroups aggregate CityObjects and furthermore are defined as special CityObjects. This implies that a group may become a member of another group realizing a recursive aggregation schema. Since CityObjectGroup is a feature, it has the optional attributes class, function and usage. The class attribute allows a group classification with respect to the stated function and may occur only once. The function attribute is intended to express the main purpose of a group, possibly to which thematic area it belongs (e.g. site, building, transportation, architecture, unknown etc.). The attribute usage can be used, if the object’s usage differs from its function. The attributes class, function and usage are specified as gml:CodeType. The values of these properties can be enumerated in code lists.

Each member of a group may be qualified by a role name, reflecting the role each CityObject plays in the context of the group. Furthermore, a CityObjectGroup can optionally be assigned an arbitrary geometry object. This may be used to represent a generalised geometry generated from the member’s geometries. The parent association linking a CityObjectGroup to a CityObject allows for the modelling of generic hierarchical groupings. This concept is used, for example, to represent storeys in buildings. See Fig. 3.1 for the simplified UML diagram.