Integrating BIM and GIS for 3D City Modelling The Case of IFC and CityGML

Abstract: 3D geoinformation has become a base for an increasing number of today’s applications. Examples of these applications are: city and urban planning, real estate management, environmental simulation, crisis and disaster management, telecommunication, facility management and others. 3D city models are presently scattered over different public and private sectors in different systems, different conceptual models, different data formats, different data schemas, different levels of detail and different quality. In addition, the potential of 3D models goes beyond visualisation of 3D objects of virtual scenes to real 3D city models. In such an environment, integration of different sources of data for building real 3D city models becomes more difficult. 3D city models are of two types, design and real world models. Design models are usually used for building industry purposes and to fulfil the requirements of maximum level of detail in the architecture, engineering and construction (AEC) industry. Real world models are geospatial information systems that represent spatial objects around us and are largely represented in GIS applications. Research efforts in the AEC industry resulted in Building Information Modelling (BIM), a process that supports information management throughout buildings’ lifecycle and is increasingly widely used in the AEC industry. Results of different integration efforts of BIM and geospatial models show that only 3D geometric information does not fulfil the integration purpose and may lead to geometrical inconsistency. Further complex semantic information is required. Therefore, this thesis focuses on the integration of the two most prominent semantic models for the representation of BIM and geospatial objects, Industry Foundation Classes (IFC) and City Geography Markup Language (CityGML), respectively. In the integration of IFC and CityGML building models, substantial difficulties may arise in translating information from one to the other. Professionals from both domains have made significant attempts to integrate CityGML and IFC models to produce useful common applications. Most of these attempts, however, use a unidirectional method (mostly from IFC to CityGML) for the conversion process. A bidirectional method can lead to development of unified applications in the areas of urban planning, building construction analysis, homeland security, etc. The benefits of these unified applications clearly appear at the operational level (e.g. cost reduction, unified data-view), and at the strategic level (e.g. crisis management and increased analysis capabilities). For a bidirectional method, a formal mapping between both domains is required. Researchers have suggested that harmonising semantics is currently the best approach for integration of IFC and CityGML. In this thesis, the focus is therefore on semantic integration of IFC and CityGML building models for bidirectional conversion. IFC and CityGML use different terminologies to describe the same domain and there is a great heterogeneity in their semantics. Following a design research method, the thesis proposes a more expressive reference ontology between IFC and CityGML semantic models. Furthermore, an intermediate unified building model (UBM) is proposed between IFC and CityGML that facilitates the transfer of spatial information from IFC to CityGML and vice versa. A unified model in the current study is defined as a superset model that is extended to contain all the features and objects from both IFC and CityGML building models. The conversion is a two-steps process in which a model is first converted to the unified model and then to the target model. The result of the thesis contributes, through the reference ontology, towards a formal mapping between IFC and CityGML ontologies that allows bidirectional conversion between them. Future development of the reference ontology may be seen as the design of a meta-standard for 3D city modelling that can support applications in both domains. Furthermore, the thesis also provides an approach towards a complete integration of CityGML and IFC through the development of the UBM. The latter contribution demonstrates how different classes, attributes and relations have been considered from IFC and CityGML in the building of the UBM. To illustrate the applicability of the proposed approach, a hospital building located in Norrtälje City, north of Stockholm, Sweden, is used as a case study. The purpose of the case study is to show how different building elements at different levels of detail can be constructed. Considering future research possibilities, the integration approach in the thesis is seen as a starting-point for developing a common database that formulates a UBM’s platform. With such a platform, data from IFC and CityGML can be automatically integrated and processed in different analyses. Other formats can also be included in further steps. Finally, the proposed approach is believed to need future research beyond the building models alone and on an implementation process for testing and verification.