CityGML Application Domain Extension (ADE): overview of developments <s> Disadvantages of ADE and open questions <s> Interoperability between building information models (BIM) and geographic information models has a strong potential to bring benefit to different demands in construction analysis, urban planning, homeland security and other applications. Therefore, different research and commercial efforts have been initiated to integrate the most prominent semantic models in BIM and geospatial applications. These semantic models are the Industry Foundation Classes (IFC) and City Geography Markup Language (CityGML) respectively. However, these efforts mainly: a) use a unidirectional approach (mostly from IFC to CityGML) for converting data, or b) Extending CityGML by conceptual requirements for converting CityGML to IFC models. The purpose of this paper is to investigate the potential of unidirectional conversion between IFC and CityGML. The different IFC concepts and its corresponding concepts in CityGML is studied and evaluated. The investigation goes beyond building objects, also including other concepts that are represented implicitly in building schemas such as building objects relations, hierarchies of building objects, appearance and other building characteristics. Due to the large semantic differences between IFC and CityGML standards, the schema mapping is based on a manual pragmatic approach without automatic procedures. The mappings are classified into three categories, namely ‘full matching’, ‘partial matching’ and ‘no matching’. The result of the study shows that only a few concepts are classified as ‘direct matching’, a few as well as ‘no matching’ while most of the building concepts are classified as ‘partial matching’. It is concluded that unidirectional approaches cannot translate all the needed concepts from both IFC and CityGML standards. Instead, we propose a meta-based unified building model, based on both standards, which shows a high potential for overcoming the shortages of the unidirectional conversion approaches. <s> BIB001 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Disadvantages of ADE and open questions <s> Abstract The Building Information Modeling (BIM) domain and the Geographic Information System (GIS) domain share a mutual need for information from each other. Information from GIS can facilitate BIM applications such as site selection and onsite material layout, while BIM models could help generate detailed models in GIS and achieve better utility management. The mapping between the key schemas in the BIM domain and the GIS domain is the most critical step towards interoperability between the two domains. In this study, Industry Foundation Classes (IFC) and City Geography Markup Language (CityGML) were chosen as the key schemas due to their wide applications in the BIM domain and the GIS domain, respectively. We used an instance-based method to generate the mapping rules between IFC and CityGML based on the inspection of entities representing the same component in the same model. It ensures accurate mapping between the two schemas. The transformation of coordinate systems and geometry are two major issues addressed in the instance-based method. Considering the difference in schema structure and information richness between the two schemas, a reference ontology called Semantic City Model was developed and an instance-based method was adopted. The Semantic City Model captures all the relevant information from BIM models and GIS models during the mapping process. Since CityGML is defined in five levels of detail (LoD), the harmonization among LoDs in CityGML was also developed in order to complete the mapping. The test results show that the developed framework can achieve automatic data mapping between IFC and CityGML in different LoDs. Furthermore, the developed Semantic City Model is extensible and can be the basis for other schema mappings between the BIM domain and the GIS domain. <s> BIB002 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Disadvantages of ADE and open questions <s> To be used as input in most simulation and modelling software, 3D city models should be geometrically and topologically valid, and semantically rich. We investigate in this paper what is the quality of currently available CityGML datasets, i.e. we validate the geometry/topology of the 3D primitives (Solid and MultiSurface), and we validate whether the semantics of the boundary surfaces of buildings is correct or not. We have analysed all the CityGML datasets we could find, both from portals of cities and on different websites, plus a few that were made available to us. We have thus validated 40M surfaces in 16M 3D primitives and 3.6M buildings found in 37 CityGML datasets originating from 9 countries, and produced by several companies with diverse software and acquisition techniques. The results indicate that CityGML datasets without errors are rare, and those that are nearly valid are mostly simple LOD1 models. We report on the most common errors we have found, and analyse them. One main observation is that many of these errors could be automatically fixed or prevented with simple modifications to the modelling software. Our principal aim is to highlight the most common errors so that these are not repeated in the future. We hope that our paper and the open-source software we have developed will help raise awareness for data quality among data providers and 3D GIS software producers. <s> BIB003 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Disadvantages of ADE and open questions <s> The validity of 3D primitives in 3D GIS datasets is often a prerequisite for using them in simulation and decision-making software, e.g. visibility analysis, noise pollution assessment, and energy estimation. However, while agreed definitions exist (in the international standard ISO19107), most software vendors ignore them and implement simpler 3D primitives, for instance by excluding interior boundaries in surfaces and/or solid. Such limitations prevent practitioners from exchanging and converting datasets, and thus to use these in other software and applications. I present in this paper val3dity, an open-source software to validate 3D primitives according to the international definitions of ISO19107. Practitioners can use it directly, without limitations: its code is freely available under the GPLv3 license, both binaries and a web-application are publicly available. It takes as input several formats (including the international standard CityGML), and outputs a report that helps users identify and understand the errors. I describe some of the engineering decisions supporting val3dity, and show that it can be used to validate real-world datasets. <s> BIB004
Because they solve the shortcomings of the default CityGML data model by enhancing it for certain purposes, ADEs entail an added layer of complexity in the data model BIB001 . Virtually always, developing an ADE by default means that it cannot be entirely interpreted by existing software, which is evident from the brief review of software (Section Overview of software support for ADE). A custom schema entails development of customised software solutions for modelling, exchange, visualisation, and utilisation of ADE-enabled CityGML datasets. It appears that it is uncommon to have the ADEs supported by software as authors often stop at the development of a schema BIB002 . Furthermore, ADEs rarely seem employed in other research. Their development usually ceases soon after publication, and in many cases work does not go beyond a research paper. While our review has shown that there is a relatively large number of ADEs, it is not common to find publicly available datasets that contain an ADE, and their validation has not been subject of research BIB003 BIB004 . Even though ADEs are intended to supplement CityGML, it is important to note that the geometries must be compatible with the Geography Markup Language (GML). This might be a disadvantage for purposes such as the conversion from IFC because it involves additional forms of geometry such as constructive solid geometry (CSG)  . The current ADE documentation leaves some open question, e.g. one of these relates to the level of detail: how would it be possible to denote the LOD of a spatial property introduced by an ADE? In CityGML, the LOD is only implicitly given by the name of the spatial property which starts with lodX (e.g. bldg:lod2MultiSurface). However, the CityGML specification does not elaborate how this should be achieved in an ADE. Therefore, when writing ADE-enabled software such as a viewer, it is not clear how it is possible to specify which ADE features are available in which LOD. Finally, it appears that, besides the CityGML standard, there is no authoritative publication focused on the ADE concept nor a '101 guide' (which we try to at least partially solve with this paper).