CityGML Application Domain Extension (ADE): overview of developments <s> Key literature <s> Recently a national 3D standard was established in the Netherlands as a CityGML Application Domain Extension (called IMGeo). In line with the Dutch practice of modeling geo-information, the ADE is devel- oped using a model driven approach. The classes are designed in UML and automatically mapped to GML schema. The current OGC CityGML specification does not provide rules or guidance on correctly modeling an ADE in UML. This article fills this gap by studying how CityGML can be extended for spe- cific applications starting from the UML diagrams. Six alternatives for modeling ADEs in UML are intro- duced and compared. The optimal alternative is selected and applied to obtain the national 3D standard. The approach was extensively discussed with international experts, who were members of both SIG3D and other working groups. As a consequence the approach was adopted by the SIG3D, the Special Inter- est Group 3D which, among other things, work on the 3D standard CityGML in cooperation with OGC. Therefore the approach contains many issues that can be generalized and reused by future domain exten- sions of CityGML. To further support this, the article formulates a model-driven framework to model CityGML ADEs. Open issues are described in the conclusions. <s> BIB001 </s> CityGML Application Domain Extension (ADE): overview of developments <s> Key literature <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> BIB002
Among the identified literature, some documents differentiate themselves as key literature, being relevant for developers venturing into developing an ADE. The starting document is the CityGML 2.0 standard  , that not only defines the ADE concept but also contains some guidelines and the two examples shown in Section ADE literature review. Given that CityGML 3.0 will probably not introduce changes to the ADE mechanism and it is yet to be published, it is to be expected that-when it comes to the ADE mechanism-the CityGML 2.0 standard will still be relevant for some time. The guides prepared by Van den Brink et al. BIB001 focus on the development of an ADE as UML. The authors offer six alternative approaches to do so, hence it is an essential read for developing an ADE as UML. Finally, it is also relevant to highlight the paper of Yao and Kolbe BIB002 focusing on implementing ADEs in a database.