CityGML Application Domain Extension (ADE): overview of developments <s> Introduction <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> Introduction <s> In the last decades, 3D city models appear to have been predominantly used for visualisation; however, today they are being increasingly employed in a number of domains and for a large range of tasks beyond visualisation. In this paper, we seek to understand and document the state of the art regarding the utilisation of 3D city models across multiple domains based on a comprehensive literature study including hundreds of research papers, technical reports and online resources. A challenge in a study such as ours is that the ways in which 3D city models are used cannot be readily listed due to fuzziness, terminological ambiguity, unclear added-value of 3D geoinformation in some instances, and absence of technical information. To address this challenge, we delineate a hierarchical terminology (spatial operations, use cases, applications), and develop a theoretical reasoning to segment and categorise the diverse uses of 3D city models. Following this framework, we provide a list of identified use cases of 3D city models (with a description of each), and their applications. Our study demonstrates that 3D city models are employed in at least 29 use cases that are a part of more than 100 applications. The classified inventory could be useful for scientists as well as stakeholders in the geospatial industry, such as companies and national mapping agencies, as it may serve as a reference document to better position their operations, design product portfolios, and to better understand the market. <s> BIB002 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Introduction <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> Introduction <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> BIB004 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Introduction <s> Abstract Climate protection concepts for cities and regions are designed to establish CO2 emission baselines and develop measures for climate change mitigation. Up to now such concepts were based on aggregated consumption and emission data and only qualitative estimations of the effect of measures were possible. To better quantify the impact of mitigation measures, a large amount of data on the building stock is needed. Very powerful analysis possibilities for an energetic and economic evaluation of scenarios arise, if continuously growing data stock organized in geographical information systems are combined with simulation models of buildings and energy systems. In this work, 3D data models in CityGML format of the entire building stock of Ludwigsburg, a German county with 34 municipalities, were used and enriched with building’s year of construction and its function to allow an automatized quantifying the climate protection indicators. In this regard, the heating demand of each individual building in the region in the current state and after two refurbishment scenarios are calculated. In addition, the local solar photovoltaic potential is determined, as the exact size and orientation of each building surface is in the 3D model available. Besides, some new methodologies are described to better quantify the costs and benefits of CO2 mitigation strategies on a local or regional level and to support decision making. <s> BIB005
The CityGML standard  of the Open Geospatial Consortium (OGC) is the staple open data model and format for the storage and exchange of semantic 3D city models. It is used in several countries around the world for a wide spectrum of application domains BIB003 . While CityGML is intended to be a universal and application-independent geographic information model, the advent of dozens of applications and burgeoning use in different geographic contexts and software BIB002 BIB004 requires some additional information that is not readily available in the CityGML data model BIB001 . An example of such a case is the application of 3D city models in the energy domain: a method employed to estimate the energy demand of a building may necessitate specific and less common information such as building occupancy BIB005 , an attribute that CityGML does not envisage by default. The developers of CityGML have foreseen situations such as this one, and thus for this purpose two ways to support augmenting the CityGML data model beyond its initial scope are offered: through (i) generic objects and attributes, and (ii) using the Application Domain Extension (ADE) mechanism. The former method has been used in practice but it has certain disadvantages (discussed later), thus the latter *Correspondence: filip@nus.edu.sg  Department of Architecture, National University of Singapore, Singapore, Singapore Full list of author information is available at the end of the article method is preferred by developers and it is consequently the topic of this paper. ADE is a mechanism for enriching the data model with new feature classes and attributes, while preserving the semantic structure of CityGML, and it is an important component of the standard since its early versions. Its main purpose is supporting additional requirements by certain use cases, which is accomplished by specifying extensions to the data model. As this paper will show, ADE is employed for a few dozen different purposes, and in spite of being around for a decade there has not been any general publication so far focused solely on this integral and important concept of CityGML. This paper is intended to bridge that gap by giving an overview of the ADE mechanism, its developments, and discussing the current landscape of available implementations. Section CityGML ADE gives a general review of the ADE concept-mostly by condensing the relevant parts of the current version (2.0) of the CityGML standard  , while Section Modelling a CityGML ADE briefly presents different approaches to model an ADE. Section ADE literature review provides a literature review of publicly available ADEs, while Section Discussion discusses the main findings and observations. Another aspiration of the paper is to serve as a reference for the developers of new ADEs.