# A Simplified CityGML-Based 3D Indoor Space Model for Indoor Applications
## 2 Background and Related Work
Topology relationships of indoor spaces are the basis for indoor spatial queries, and the establishment of explicit topology relationships can be helpful in constructing adjacencies of indoor spaces and generating indoor paths [44]. One approach that can be attempted to establish topological relationships for indoor spaces is to simplify building components, such as walls, by reducing the thickness to a thin surface [45] and representing two adjacent rooms that share the common surface. The simplified description of a building in the literature [45] can be summarized as follows: In the simplified model, the exterior boundary surface of a building is the outer contour of the building, while the interior boundary surface is located at the center of the interior wall, as shown in Figure 4a. However, this approach cannot be applied to buildings with irregular outer contours. In Figure 4b,  
Figure 3: Representation of interior rooms in City Geography Markup Language (CityGML). Figure 2: Representation of topology by XLinks. for example, if the exterior boundary surface is still the outer contour of the building, the rooms will be unenclosed and the topological relationships of the rooms could not be represented. Furthermore, the method does not attempt to simplify the building components between stories. [PERSON] et al. [46] proposed a geometrically simplified description for the interior spaces of buildings using the prism model. However, the prism model is not applicable to the top and bottom faces of an indoor space with different 2D footprints. In addition, this method does not involve the simplification of doors and windows. Some methods [47; 48] that simplify the CityGML high-level LOD models to generate low-level LOD models only simplify the exterior characteristics of buildings and cannot be used to describe the simplification of interior structures. In summary, the existing literature indicates that the LODs concept in the current version, CityGML 2.0, is no longer sufficient for emerging indoor applications. Indoor 3D models suitable for indoor navigation and emergency rescue should be designed according to the requirements of indoor applications. On the one hand, the models are required to describe detailed indoor geometric and semantic information, but not all of the indoor details. On the other hand, considering the possible adjacency among indoor rooms, the topological relationships of rooms should be described according to the CityGML2.0 XLinks. However, there is, to the best of our knowledge, no 3D indoor model that can properly describe the geometric and semantic information of indoor spaces as well as the topological relationships of indoor spaces. Therefore, we introduce a simplified 3D indoor space model based on CityGML to aid in indoor navigation and emergency rescue. It should be noted that this research does not change CityGML descriptions for indoor spaces but attempts to suggest a 3D indoor model based on CityGML that is suitable for indoor applications, such as indoor navigation. Since CityGML3.0 is still under revision, this work still adopts the CityGML2.0 schema and UML to describe indoor spaces according to the CityGML2.0 encoding standard, while the CityGML3.0\({}^{\prime}\) LODs concept is adopted for indoor LOD design.