In various meetings and discussions SIG 3D, OGC CityGML SWG and TUM have collected several topics which are important for the further development of the CityGML standard. However, before starting with the development of a new major version of CityGML, it was regarded as useful to first discuss these topics in a workshop with a broader audience. In the following paragraphs we describe each topic in more detail and indicate why the topic is important.
Improved Support for Simulations and other Applications
On the one hand CityGML is a very useful and important source of information for different types of simulations, whereas on the other hand the results of simulations can be fed back to the original CityGML data for thematic enrichment and data fusion. Therefore, semantic 3D city models and simulations should become tighter coupled in the future.
In most simulations time plays an important role, i.e. dynamic / time-varying feature properties (spatial and thematic, e.g. electrical energy demand or production potential for a building along the course of the day / week / year), which are not yet supported in CityGML.
Furthermore the quality of individual attributes needs to be represented and propagated when different attributes are combined (e.g. multiplied). This requires the definition of qualified attributes (metadata at individual attribute level like lineage, accuracy, unit of measure, date of acquisition etc.) This concept is similar to the INSPIRE complex attributes but the definition should be done in a more systematic way.
LOD Concept of CityGML
Currently the CityGML specification defines five LODs. However, several works exist which suggest modifications or even a replacement of the current LOD concept. These include the separation of the current LODs into semantic and geometric LODs, complementing the current concept by separate indoor LODs, or introducing more / less / continuous LOD levels.
This topic induces a fundamental change to the current structure of CityGML and thus needs to be discussed thoroughly in a larger round of developers, users and data providers before any decision will be made. The alternative suggestions have to be analysed and their advantages and disadvantages over the current concept have to be identified. Only afterwards it will be possible to decide whether the current LOD concept should be kept or modified. This decision has also to take into account the opinion of the users of CityGML, i.e. if the acceptance of a new LOD concept will be given by them, if the new concept will be easy to understand and if the investments for implementing the new concept are bearable.
Extension of CityGML by new objects
CityGML only provides some basic objects and attributes. If for a certain application more specific objects and attributes are required, they can be added to CityGML by means of an ADE or by generics. However, it has been proposed to add permanent support for other types of constructions like walls, fences etc., by defining specific feature types for these objects. Furthermore, suggestions have been made to extend the Buildings module by Storeys, BuildingUnits, building components and further attributes. These concepts exist in the INSPIRE Buildings theme and they have also been requested by several CityGML users.
Other possible extensions might be utility networks (e.g. UtilityNetworkADE) and metadata at dataset level.
Another important topic is the extension of CityGML by BIM/IFC concepts. CityGML and BIM represent similar but not identical concepts. To support a smoother conversion between the two concepts, the CityGML Buildings module needs to be extended by relevant BIM concepts such as volumetric elements. However, due to the extensiveness of this topic we want to delay the discussion to a later workshop in about seven to nine months or even to organise a workshop dealing specifically with this topic only and addressing primarily the BIM community.
Stronger Harmonisation with 2D Cadastre and Models
CityGML currently does not allow 2D geometries to be added to the data. Furthermore, buildings cannot be represented by 3D points and 2,5D geometries always have to be defined with a constant z value. Due to the increasing importance of INSPIRE and the migration of national frameworks to 3D, where these geometries are admissible, it needs to be discussed whether 2D geometries are to be allowed in CityGML as well and whether the definition of LOD0 should be modified to allow unique z values and 3D points.