Digital geoTwin Vienna: Towards a Digital Twin City as Geodata Hub

Virtual 3D city models usually evolved from other geodata sets and were not set up from scratch. As traditional geodata used to be only 2D and 2.5D for a long time, 3D city models started naturally with 3D building models. On the one hand, this has a technical aspect, because buildings, especially when being modelled in a higher level of detail, can neither be described in 2D nor in 2.5D. On the other hand, this is because buildings form the identity of a city. This can clearly be seen when looking at the sights of a city, which are usually historic or modern buildings, such as cathedrals, palaces, skyscrapers, etc. In many cases, the 3D building models themselves were called 3D city model, because they are so essential for a virtual 3D city. A common way to create a 3D city model beyond the buildings is to combine them with 2D (i.e. city map) and 2.5D (i.e. digital terrain model) GIS data sets. In case the models are used for visualization purposes only, the city map is normally used in a raster format as texture on the terrain model. In case semantic 3D models are aimed for in order to use them for analysis, 2D city map vector data are raised, e.g. to a terrain model or a 3D point cloud. Continuative steps to enrich the virtual scene are often adding 3D bridge models or vegetation created by a set of template tree models and point-based tree information with height and tree top diameter as attributes to the point information. Problems usually occur due to temporal incoherence of the data sets. While for visualization purposes, these problems might be neglectable, they have to be tackled in case the resulting 3D city model should serve as basis for a city information model. In this paper, we want to discuss a new strategy in producing the 3D city model as well as other geodata products of Vienna which completely rethinks and reverses the geodata workflows currently in use. The centre of the strategy is to use the existing three-dimensional surveying and mapping data and potentially further input data to directly model a Digital geoTwin-a virtual, semantic 3D replica of all elements and objects of the city. Digital twins are an upcoming concept of digitizing elements, processes and systems of physical entities in order to create living digital simulation models as collaborative platform for many disciplines. We chose to add the prefix geo to our neologism Digital geoTwin to emphasize our focus on the geodetic, geometric aspect of creating semantic geo-objects for a digital twin. This Digital geoTwin should allow to derive other needed GIS data sets from it, which in corollary ensure full temporal coherence for all derived products. By linking the objects of the Digital geoTwin with further data and information, e.g. census data, socio economic data, energy consumption data, maintenance management data, etc., a city information model can be built up to serve as basis for a living digital twin of Vienna.