# A Simplified CityGML-Based 3D Indoor Space Model for Indoor Applications
## 3 Simplified 3D Indoor Space Model (3 DSISM) Based on CityGML
### Simplified Representation for Indoor Spaces
#### 3.2.2 Simplified Representation for Building Components and Indoor Spaces
After simplification, one side of the surfaces between adjacent rooms upstairs and downstairs may be associated with two or more rooms, as shown in Figure 13. Considering the principle of topological consistency, the simplification of horizontal building components should meet the maximal segmentation so that there is only one room on each side of a surface. XLinks is adopted to describe the topological relationships of adjacent rooms. Figure 11: Simplification of ceilings and floors. Figure 10: Representation of roofs, ceilings, floors and ground plates in CityGML. * Descriptions of doors and windows  
Doors and windows are building elements that connect functional spaces. In CityGML, doors and windows belong to the class _Opening_, with geometric properties represented by _Polygon_. Doors and/or windows are located in the center of an interior and exterior wall, or two interior walls, and are connected with walls to form a composited surface, as shown in Figure 13(a). When a wall is simplified, a part of the walls is removed (the white part in Figure 13(a)), and a door and/or a window are connected to the simplified interior wall to form a composited surface. In general, a window is in an interior wall, and the boundary of the window is an inner ring of the interior wall; therefore, the window (Figure 13(b)) is a hole in the interior wall. A door (Figure 13(c)) is connected to an interior wall at common edges and is not a hole in the interior wall. Semantically, doors and windows maintain their original semantic properties and are objects of the class _Opening_. * Simplification and description of indoor functional spaces
* Description of Rooms and Corridors (or halls)  
Figure 14: Representation of doors and windows. (**a**) Representation of a window in CityGML. (**b**) Simplified representation of a window. (**c**) Simplified representation of a door. Figure 8.0 Maximal segmentation of surfaces. After the simplification of the building components above, it is possible to enclose indoor functional spaces, such as rooms and corridors, with the boundary surfaces. The indoor spaces are represented by _Solid_ geometrically and as _Room_ semantically with function, class and use [22] to specify a room or a corridor. A corridor is an open space that horizontally connects rooms and staircases. In general, a virtual surface, _ClosureSurface_, can be used as a common boundary surface between a corridor and a staircase to enclose spaces together with other boundary surfaces and to connect a corridor and a staircase. 2. Simplification and description of stairs  
A stairs is a functional space connecting two adjacent stories. Stairs belong to the class _IntBuildingInstallation_ in CityGML, of which the location and shape are represented in detail by _GM_MultiSurface_. However, indoor navigation and emergency rescue are not concerned with the detailed shape of a stairs but more with the indoor spaces connected to the stairs and the operation state of the stairs. Therefore, in this research, the shape, structure, and other details of a stairs are ignored, and it is described as a functional space enclosed by interior walls as well as virtual surfaces, and assigned _Room_ as semantic property with function, class, and use, as shown in Figure 8.1