## 4 Discussion and Conclusions
For example, wheelchair-enabled access can be derived from a sequence of _IfcSlab_ and _IfcRamp_ objects but also from the existence of the _IfcTransportElement_ (of type elevator). At the top level of the hierarchy, _IfcSite_ from IFC maps to _SITE_ frame from NeTEx, _IfcBuilding_ maps to STOP PLACE, a _QUALY_ within a _STOPLACE_ maps to _IfcSpace_, _ACCESS SPACE_ leading to a _STOP PLACE_ also maps to _IfcSpace_, a STOP PLACE ENTRANCE to some _STOP PLACE_ maps to _IfcDoor_, a _RAMP EQUIPMENT_ leading to a _STOP PLACE ENTRANCE_ maps to _IfcRamp_. NeTEx can accept the attributes of the IFC objects such as name, description, location, and dimensions (width, height, length). The mapping principle obtained from the railroad station use case can be automated as soon as passenger information systems have access to digital building-data sets. The building data extracted from BIM (IFC) models can complement the transportation information data in NeTEx models. In the current literature, we could not find any interoperability framework for these two domains. The proposed concept therefore suggests that it is possible to reuse the models of BIM to feed the models of transportation infrastructure, and provides the mapping. The technological basis for interoperability is based on open international standards such as IFC, Transmodel, and NeTEx, which are widely accepted in both the construction and transportation sectors. As proposed in [19], automatic conversion of data conforming to different specifications is a key to interoperability between heterogeneous systems. In addition to harmonising data between different technologies used in transportation, our work extends existing efforts to integrate services offered by transportation agencies with new data sources that additionally support navigation. When planning the proposed transformation, some problems also arise. Our approach relies on given IFC objects, which do not always have to be part of an IFC instance model. For example, sometimes _IfcSpace_ objects are missing from the model because a user/modeller did not use them. Since spaces represent nodes of the navigation-network topology, in such cases it is not possible to extract a topology from the IFC using the proposed method. Similar problems can occur with stairs, ramps or door types that represent relationships between nodes. For example, the door type is important for people with disabilities, since the direction in which the door is pushed can be crucial for passage. In addition, in practise, the modelling of ramps or stairs is sometimes ambiguous (or incorrect) and it is difficult to identify such building elements and their connections to spaces from the IFC model. Previous research has also shown that it is difficult to automatically extract vertical connections from IFC due to different approaches to modelling the building [21]. To overcome this problem, the Strug-Slusarczy algorithm that extracts horizontal and vertical connections can be applied. The method covers well-modelled aspects of vertical-path identification. However, this remains an open question, since building modelling allows ambiguous expression of connection of building elements. At this stage, missing spatial access points representing stairs or ramps could be added manually if the transcoding software provides such a feature. At the conceptual level presented in this paper, we focused on the most important elements of the buildings. Therefore, we have used only a small amount of available information. Usually, detailed models of fixed infrastructure provide more information about the buildings and the available equipment. Transmodel provides reference definitions for transportation infrastructure such as platforms, waiting rooms, access areas, ticket machines, seating areas, boarding areas, etc. These parts could also be available in the IFC model. However, there is no standard method for modelling and coding these details in IFC. Therefore, automatic extraction of this information is difficult, if not impossible, at this time.