3.1. UML database design¶

CityGML is a common information model for 3D urban objects and provides a comprehensive and extensible representation of the objects. It is explained in detail in the CityGML specification [GKNH2012], [GKCN2008] and [Kolb2009]. Most thematic classes are (transitively) derived from the basic classes Feature and FeatureCollection, the basic notions defined in ISO 19109 and GML3 for the representation of features and their aggregations. Features contain spatial as well as non-spatial attributes, which are mapped to GML3 feature properties with corresponding data types. Geometric properties are represented as associations to the geometry classes described in Section 3.1.3.1. The thematic model also comprises different types of interrelationships between Feature classes like aggregations, generalizations, and associations.

The aim of the explicit modelling is to reach a high degree of semantic interoperability between different applications. By specifying the thematic concepts and their semantics along with their mapping to UML and GML3, different applications can rely on a well-defined set of Feature types, attributes, and data types with a standardised meaning or interpretation. In order to allow also for the exchange of objects and/or attributes that are not explicitly modelled in CityGML, the concepts of GenericCityObjects and GenericAttributes have been introduced.

This chapter discusses the mapping of the CityGML data model to a general database design for the 3D City Database on a conceptual level using UML diagrams. The following pages cite several parts of the CityGML specification which are necessary for a better understanding. Main focus is put on explaining the simplifications and customizations of the CityGML data model and the resulting differences to the 3D City Database design. Design decisions in the model are explicitly visualised within the UML diagrams.

Note

For intuitive understanding, classes that are merged to a single table in the relational schema are shown as orange blocks in the UML diagrams. n:m relations between UML classes that require an additional table are represented as green blocks.