CityGML Application Domain Extension (ADE): overview of developments <s> Overview of software support for ADE <s> Abstract CityGML is the international standard of the Open Geospatial Consortium (OGC) for the representation and exchange of 3D city models. It defines the three-dimensional geometry, topology, semantics and appearance of the most relevant topographic objects in urban or regional contexts. These definitions are provided in different, well-defined Levels-of-Detail (multiresolution model). The focus of CityGML is on the semantical aspects of 3D city models, its structures, taxonomies and aggregations, allowing users to employ virtual 3D city models for advanced analysis and visualization tasks in a variety of application domains such as urban planning, indoor/outdoor pedestrian navigation, environmental simulations, cultural heritage, or facility management. This is in contrast to purely geometrical/graphical models such as KML, VRML, or X3D, which do not provide sufficient semantics. CityGML is based on the Geography Markup Language (GML), which provides a standardized geometry model. Due to this model and its well-defined semantics and structures, CityGML facilitates interoperable data exchange in the context of geo web services and spatial data infrastructures. Since its standardization in 2008, CityGML has become used on a worldwide scale: tools from notable companies in the geospatial field provide CityGML interfaces. Many applications and projects use this standard. CityGML is also having a strong impact on science: numerous approaches use CityGML, particularly its semantics, for disaster management, emergency responses, or energy-related applications as well as for visualizations, or they contribute to CityGML, improving its consistency and validity, or use CityGML, particularly its different Levels-of-Detail, as a source or target for generalizations. This paper gives an overview of CityGML, its underlying concepts, its Levels-of-Detail, how to extend it, its applications, its likely future development, and the role it plays in scientific research. Furthermore, its relationship to other standards from the fields of computer graphics and computer-aided architectural design and to the prospective INSPIRE model are discussed, as well as the impact CityGML has and is having on the software industry, on applications of 3D city models, and on science generally. <s> BIB001 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Overview of software support for ADE <s> The VI-Suite is a free and open-source addon for the 3D content creation application Blender, developed primarily as a tool for the contextual and performative analysis of buildings. Its functionality has grown from simple, static lighting analysis to fully parametric lighting, shadowing, and building energy analyses. It adopts a flexible, mesh geometry based approach to the specification of calculation points and this has made it suitable for certain types of 3D geospatial analyses and data visualisation. As this is the first academic paper to discuss the VI-Suite, a history of its development is presented along with a review of its capabilities of relevance to geospatial analysis. As the VI-Suite combines the functionality of 3D design software with performance simulation, some of the benefits of this combination are discussed including aspects that make it suitable for the processing and analysis of potentially large geospatial datasets. Example use cases with a 3D city model of the Hague are used to demonstrate some of the geospatial workflows possible and some of the result visualisation options. The free and open-source nature of the VI-Suite, combined with the use of Blender mesh geometry to define calculation points, has encouraged usage scenarios not originally intended by the authors, for example large scale urban shadow and radiation analyses. The flexibility inherent in this mesh based approach enables the analysis of large geospatial datasets by giving the user refined control over the distribution of calculation points within the model. The integration of GIS analysis into a digital design package such as Blender offers advanced geometry/material editing and specification, provides tools such as ray casting and BVH tree generation to speed up the simulation of large datasets, and enhanced visualisation of GIS simulation data including animated city fly-throughs and high quality image production. The VI-Suite is part of a completely open-source tool chain and contributions from the community are welcome to further enhance its current geospatial data capabilities. <s> BIB002 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Overview of software support for ADE <s> As a domain extendable 3D city model the OGC standard CityGML has been increasingly employed as a dominant consensus over different application areas. An essential challenge encountered in many research and commercial activities in the field of 3D city modelling is to find a reliable approach for achieving high-efficient storage and management of data models according to CityGML with its Application Domain Extensions (ADE) in order to ensure interoperable data access across broad application domains. Based on graph transformation, this paper introduces a new approach along with an extensive database structure that allows for dynamically extending the spatially-enhanced relational databases for handling arbitrary CityGML ADEs by means of graph transformation systems. With this approach, relational database schemas with simplified and optimized table structures can be automatically generated from the XML application schemas of CityGML ADEs by performing a series of user-defined graph transformation rules which can describe complex mapping rules for transforming object-oriented data models to relational database models in a fully declarative way. The proposed approach has been successfully implemented and tested based on a number of different CityGML ADEs like Energy ADE, Dynamizer ADE, and UtilityNetwork ADE. <s> BIB003 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Overview of software support for ADE <s> Over the last decade, more and more cities and even countries worldwide are creating semantic 3D city models of their physical environment based on the international CityGML standard issued by the Open Geospatial Consortium (OGC). CityGML is an open data model and XML-based data exchange format describing the most relevant urban and landscape objects along with their spatial and non-spatial attributes, relations, and their complex hierarchical structures in five levels of detail. 3D city models, which are structured according to CityGML, are often used for various complex GIS simulation and analysis tasks, which go far beyond pure 3D visualization. Due to the large size and complexity of the sometimes country-wide 3D geospatial data, the GIS software vendors and service providers face many challenges when building 3D spatial data infrastructures for realizing the efficient storage, analysis, management, interaction, and visualization of the 3D city models based on the CityGML standard. Hence, there has been strong demand for an open and comprehensive software solution that can provide full support of the aforementioned functionalities. The ‘3D City Database’ (3DCityDB) is a free 3D geo-database solution for CityGML-based 3D city models. 3DCityDB has been developed as an Open Source and platform-independent software suite to facilitate the development and deployment of 3D city model applications. The 3DCityDB software package consists of a database schema for spatially enhanced relational database management systems (ORACLE Spatial or PostgreSQL/PostGIS) with a set of database procedures and software tools allowing to import, manage, analyze, visualize, and export virtual 3D city models according to the CityGML standard. Within this paper, the software suite is illustrated and explained in detail with respect to the related technical implementations and the underlying conceptual software design. Moreover, the utilization of 3DCityDB in different projects and practical application fields are also presented in this paper. <s> BIB004
Nowadays a number of software packages support CityGML, from data generators to visualisers. In this Fig. 5 Excerpt of the UML diagram of the 3D Metadata ADE, depicting how to extend it to incorporate the metadata related to other ADEs (such as Noise ADE) i.e. an ADE of an ADE for an ADE. Source: 3D Metadata ADE (https://github.com/tudelft3d/3D_Metadata_ADE) section they are overviewed considering their ADE support. The most current list of maintained software with CityGML support is the one available at the official CityGML website BIB002 , and hence it is taken here as the authoritative list for this analysis. A quick inspection of the software revealed that support for ADEs is limited. Introducing a new ADE entails modifying an existing software package or developing a new one. Most software packages have no support for ADEs whatsoever, hence the continuation of this section focuses on the software that at least partially has a support for ADEs. The free CityGML geodatabase solution 3DCityDB currently does not offer full support for ADEs. However, there is work in progress in extending 3DCityDB supporting CityGML ADEs BIB003 BIB004 . Furthermore, there are some independent implementations demonstrating support for ADEs in 3DCityDB, e.g. EnergyADE, Dynamizer ADE, and UtilityNetworkADE. FZK Viewer, a free Windows-only visualiser supports visualisation of ADE data. It is a schema-aware software and thus supports any ADE as any other GML application schema. It requires the ADE XML schema to process the new objects and attributes defined in the ADE. citygml4j, the open source Java API for CityGML, provides full support for handling ADEs. It is capable of reading ADE-enriched CityGML datasets and writing the ADE classes/attributes back to disk without loss of information. azul, a free macOS visualiser supports the IMGeo (reviewed in Section ADE literature review), which is an ADE. Other than this ADE, the software may visualise geometries that are inside any ADE extension. However, thematic extensions concerning features (e.g. new attributes) that are not predefined in the code are not shown. The software is open-source, hence support for other ADEs is a viable option. 3dfier BIB001 , an open-source software to automatically construct 3D city models in OBJ and CityGML formats, also implements the IMGeo ADE 2.1.1. FME, the commercial extract, transform, load (ETL) software, can manipulate ADE-enabled CityGML datasets as long as the XSD schema is provided. Figure 6 shows an example of a dataset with Noise ADE attributes being visualised in the FME Data Inspector. On a related note, the ADE mechanism is currently being implemented in CityJSON, a JSON-based implementation of CityGML.