Semantic 3D city models describe city entities by objects with thematic and spatial attributes and their interrelationships. Today, more and more cities worldwide are representing their 3D city models according to the CityGML standard issued by the Open Geospatial Consortium (OGC). Various application areas of 3D city models such as urban planning or architecture require that authorities or stakeholders manage parallel alternative versions of city models and their evolution over time, which is currently not supported by the CityGML standard 2.0. In this paper, we propose a concept and a data model extending CityGML by denoting versions of models or model elements as planning alternatives. We support transitions between these versions to manage history or evolution of the city models over time. This approach facilitates the interoperable integration and exchange of different versions of a 3D city model within one dataset, including a possibly complex history of a repository. Such an integrated dataset can be used by different software systems to visualize and work with all the versions. The versions and version transitions in our proposed data model are bi-temporal in nature. They are defined as separate feature types, which allow the users to manage versioning and to perform queries about versions using an OGC Web Feature Service. We apply this data model to a use case of planning concurrent versions and demonstrate it with example instance data. The concept is general in the sense that it can be directly applied to other GML-based application schemas including the European INSPIRE data themes and national standards for topography and cadasters like the British Ordnance Survey Mastermap or the German cadaster standard ALKIS.

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Many of us have an intuition that we should be able to extend the technical and commercial success of road navigation in large geographic spaces to smaller spaces such as parks, shopping malls, business estates, airports, train stations, crime scenes, disaster sites, and individual buildings. Designing real applications leads to three key technical and business questions:"What are the requirements?" There are clear differences in comparison with road navigation. Smaller spaces have a human-scale level structure embedded in 3-dimensional space. Visualisation and analysis can be as important as navigation itself. Does the turn-by-turn guidance model still make sense?"Where do models come from?" Smaller spaces have complex structure that are complex and can change frequently. Multiple sources inevitably have semantic and quantitative inconsistencies. How do you find content? How do you integrate content? How do you update content?"How do you locate a mobile device in smaller spaces?" Compared to road navigation, the precision requirements are tighter, the difficulties in radio propagation are extreme, and level information within structures is essential.Consideration of these questions in a practical application at the Italian fire training centre at Montelibretti provides some answers.