# A Simplified CityGML-Based 3D Indoor Space Model for Indoor Applications
## 2 Background and Related Work  
The building module of CityGML 2.0 gives different detailed descriptions of the same building using LOD0 to LOD4, as shown in Figure 1. A building is represented as a footprint by a horizontal 3D surface in LOD0. LOD1 is represented as coarse a block-shaped model by a solid or multisurface. LOD2 adds simplified roof structures to LOD1, and the outer surfaces of a building can be represented semantically by thematic features. Openings (windows, doors) and other detailed exterior structures are added to LOD2 to obtain the most accurate exterior representation: LOD3. In LOD4, detailed interior building structures, which are composed of rooms probably with furniture, can be modeled upon LOD3. The LODs concept of CityGML2.0 allows descriptions for the same features at different levels of detail. This allows handling of the complexities of urban models and facilitates spatial analysis and visualization [25]. However, the concept also creates some problems. For example, the interior spatial structure of a building can only be described in LOD4, which means that the detailed indoor information can only be described based on LOD3, which is the most detailed exterior shape of a building. This definition somewhat hinders indoor applications, such as indoor navigation, which require access to detailed indoor space information but not precise exterior building information. There have been a number of efforts to refine the LODs concept of CityGML2.0. Typical improvements include the progressive definitions of LODs for geometric and semantic levels of detail [38; 39], the progressively finer levels of detail descriptions for indoor and outdoor spaces [40; 41], and mapping the LOD4 descriptions of the current version for indoor spaces to four levels of detail, LOD0 to LOD3, to describe indoor space information [25; 38]. Consequently, in the upcoming version, CityGML 3.0, LOD4 will be removed, and all objects (including the interior of a  
Figure 1: Five representations of a building in level of detail (LOD)0 to LOD4 [37]. building) can be represented in LOD0 to LOD3. For example, it is possible to represent the interior structure of a building in LOD3 with a coarse exterior shell in LOD1. The new LODs concept greatly increases the flexibility of interior space descriptions. Therefore, LODs can be tailored according to different indoor applications without having to limit the interior space information to only one level of detail. While there is a large amount of literature that focuses on improving the geometric and semantic LODs of the current version, limited literature has reported on the topological description of CityGML for interior spaces. In CityGML2.0, a geometric object shared by other geometric objects can be referenced by a GML geometry attribute xlink: href. Figure 2 shows that two solids (s1 and s2) share a common surface (surface1, blue). Hence, the geometry of surface1 is represented only once and referenced by the two solids through XLinks. The topology representation of XLinks is limited to the features that have a shared geometry object. However, the XLink topology is not used to describe interior spaces that share a common building element in CityGML. In Figure 3, although room1 is adjacent to room2 and wall w1 is the common element, CityGML uses the two sides of the wall (thick wall) to represent the boundary surfaces of the two rooms, respectively, yielding two separate indoor spaces without using XLinks to describe the shared common element: The interior wall. Not only does this description for rooms have some data redundancy, but it does not represent the topological relationship of the rooms. Likewise, the topological relationship of the adjacent rooms upstairs and downstairs is also not represented. Moreover, the topological relationship description is not specified in the schema and Unified Modeling Language (UML) document of CityGML 3.0 [42; 43], which is under development.