## 1 Introduction
### Related Work
#### 1.1.4 Interoperability, Spaces, and Navigation  
For building data modelling to become a common resource for stakeholders in building projects and beyond, various levels of interoperability should be established. At a minimum, technical and semantic interoperability must be in place to share building data [14; 15]. The use of the IFC standard beyond the construction industry contributes to broader organisational interoperability. Applying such an approach is the goal of this work, where we seek to reuse building information models to provide transportation models with data about physical infrastructure. In the transportation sector, navigation is a particularly important component of trip planning. In addition to the obvious task of finding appropriate routes and making transport-mode choices, more detailed navigation is often required at facilities where passengers board, alight, or change transport modes. These facilities are usually buildings or other infrastructure facilities in the built environment. The IFC models contain geometric and spatial information about the buildings and their parts. The building parts are connected to each other, and the topological relationships are stored in the model. In this work, we propose an interoperability framework that supports mapping of physical infrastructure information between BIM and Transmodel. The main goal of such interoperability is to inform navigation algorithms. Therefore, the spatial topology of the built environment is the main object of the mapping. Our proposal to bridge these two worlds is to extract the topology of the building spaces from the IFC model and translate this topology into a specification compatible with Transmodel, which in our case is the NeTEx model. The topology represents the navigation network within the building and is a common abstract model shared by the both domain-specific models. This approach is common in the harmonisation of domain-specific models in the transportation domain [19]. Applications of BIM outside the construction sector, such as mapping BIM data to GIS, also use a similar methodology [20]. Unique to our work is the application of the principle to achieve interoperability between construction and transportation models. The automatic creation of navigation-data models and navigation networks from IFC is not new. [PERSON] et al. [21] have compiled a review of extensive previous work in this area. The research shows that the IFC-based description of indoor spaces provides enough information about spaces for the creation of navigation networks, either by using IFC directly or by developing extended data models based on the IFC schema. To fully support navigation within a fixed infrastructure, a navigation-network model should be created to cover the building space, wayfinding should be developed, and localisation methods should be available. In our research, we focused on the first task, the creation of a building-space model that provides enough information for passenger wayfinding and routing. From this abstract navigation model, we focused on the preparation of a transportation model that includes fixed infrastructure. Wayfinding and routing are not part of our work, since the Transmodel-based journey planners support the routing procedures implemented in the journey planners \"on-the-fly\". Because the IFC schema is extensive, with over 600 classes, an IFC model can contain detailed information about various aspects of a building. Much of this information is not useful for navigation. By comparing research results from different authors, [PERSON] et al. [21] extracted a subset of IFC classes that is sufficient for creating navigation networks. According to the authors, the most important components for indoor navigation are the rooms and building elements such as walls, doors, and windows that define the connection between rooms. Based on these elements, information can be obtained and represented in navigation models. In general, there are two approaches or methods for creating navigation models.