# A Simplified CityGML-Based 3D Indoor Space Model for Indoor Applications
## 1 Introduction  
Smart cities, as a new urban development concept, extend people's perceptions and explorations of spatial information from outdoor spaces to indoor spaces [1; 2]. In particular the continuous development of indoor positioning technology has given rise to many indoor space applications, such as indoor navigation, indoor emergency rescue, and indoor path planning [3; 4; 5; 6]. With the help of various indoor positioning technologies [7; 8; 9; 10; 11; 12], people can easily get from one place to another in an indoor environment. In the event of an indoor emergency, such as a fire, rescuers are able to quickly develop evacuation routes to protect life and property to the greatest extent possible. However, the increasing complexity of high-rise buildings has also led to an increasingly complex indoor space environment. The complexity of indoor spaces is partly a barrier to efficient indoor navigation and rapid emergency response but also highlights the importance of indoor navigation and emergency rescue. Unlike outdoor spaces, people are exposed to various obstacles, such as walls and indoor facilities, when passing through indoor spaces. Therefore, indoor navigation requires detailed information about indoor spaces, such as the location of indoor spaces and the spaces adjacent to an indoor space. In this context, a three-dimensional model that can accurately describe the complex environment of an indoor space in detail is indispensable [13]. A 3D model that describes the geometric, semantic, and even topological information of a building's indoor space plays an important role in indoor spatial inquiry, spatial analysis, and thegeneration of path information. In terms of 3D models for buildings, the well-known Building Information Modeling (BIM) [14] can describe the geometric and semantic information of buildings in detail. As the primary data exchange standard of BIM, the Industry Foundation Classes (IFC) standard developed by buildingSMART [15] describes geometric and semantic information of building components for indoor spaces with a variety of classes, such as IfcWall, IfcSlab, and IfcSpace, which can serve as the basis for indoor applications, such as indoor navigation and emergency rescue [4; 16]. However, the building elements in IFC are mostly described by Swept Solid and Constructive Solid Geometry (CSG), while only a small portion of the elements are described by Boundary Representation (Brep), which is widely used in the GIS domain [17; 18]. On the other hand, although BIM models provide rich information about buildings, they do not have the spatial analysis capabilities [19; 20]. Therefore, in most cases, BIM models are used as an ideal source of 3D data for conversion to GIS data, such as converting BIM data in IFC to 3D GIS data in CityGML [17]. City Geography Markup Language (CityGML) [21] from the Open Geospatial Consortium (OGC) is an open data model used for the representation and exchange of 3D city models. In the current version, 2.0 [22], CityGML adopts five different levels of detail (from coarse LOD0 to highly detailed LOD4) to represent various features in city models, ranging from a single building to a whole city. Thanks to the concept of LODs, CityGML has a wide range of applications, such as indoor navigation and emergency rescue [23; 24]. The building model of CityGML is able to represent the thematic and spatial aspects of buildings in LOD0 to LOD4. In LOD4, geometric and semantic information about the interior spaces of a building are described in detail. Geometrically, Brep is used to represent the geometry of building components. For example, floors, ceilings and interior walls are represented by surfaces and a room is represented by a solid, which semantically correspond to FloorSurface, CeilingSurface, InteriorWallSurface and Room, respectively. Topologically, CityGML adopts the XML concept of XLinks to represent a common geometrical object shared by two solids. The XLink topology is nearly as powerful as the explicit GML3 topology but simpler and more flexible.