# 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  
Different buildings have different indoor spaces and components types. In this work, the representative building components and indoor functional spaces are selected to be simplified and described according to the requirements of indoor navigation, following the principles of simplification. * Simplification and description of vertical building components  
A wall separates outdoor and indoor spaces in the vertical direction, as well as different indoor rooms. In this work, the vertical components refer to walls. In CityGML, the walls separating outdoor and indoor spaces are described as walls and interior walls, respectively, while the walls separating different rooms are described as interior walls, as shown in Figure 7.  
Considering the requirements of indoor navigation and the complexities of building outer contours (Figure 4b), in this work, all the walls (thick walls) of a building are represented as surfaces and are replaced by their center surfaces, according to the simplification principles, as shown in Figure 8. The simplified walls are geometrically represented by geometric primitive _Polygon_. Semantically, the walls separating outdoor and indoor spaces are simplified from exterior and interior walls to a single wall, resulting in semantic inconsistency. Since the indoor space model describes information about indoor spaces, the simplified wall is semantically described as _InteriorWallSurface_. In addition, the layouts of rooms in the same story are not exactly the same. After simplification, one side of a wall may be associated with two or more rooms, as shown in Figure 9. Considering the principle of topological consistency, the simplified wall should meet maximal segmentation so that there is only one room on each side of a wall. Two adjacent rooms share the wall, and XLinks is used to describe the topological relationships. \(\bullet\) Simplification and description of horizontal building components  
In CityGML, ceilings and floors separate different stories horizontally, roofs separate indoor spaces from outdoor space, and ground plates connect buildings with the ground, as shown in Figure 10. In this research, ceilings, floors, roofs, and ground plates are classified as horizontal building elements. Figure 8: Simplification of walls. Figure 7: Representation of walls in CityGML. Floors and ceilings separating adjacent stories, according to the principles above, are simplified and replaced by center surfaces and represented geometrically by geometric primitive _Polygon_. For two adjacent rooms upstairs and downstairs, the floor of one room is the ceiling of the other room after simplification, and both floor and ceiling are the same feature, resulting in a semantic inconsistency. Therefore, the simplified surface can be assigned either _FloorSurface_ or _CeilingSurface_ semantically but not both (as shown in Figure 11). Indoor activities are in the story spaces between floors and ceilings, without being concerned with roofs and ground plates; however, when simplifying these building elements, irregular building shapes should be considered as much as possible. As shown in Figure 12a, the ground plates of the building are not on a horizontal plane. When the floor and ground plate of room C are simplified after simplifying the ceiling and floor between room A and room B with a center surface, there would be a geometric error if the ground plate was ignored and if only the floor of room C was retained. Therefore, floors and ground plates are also simplified with center surfaces. The simplified surface can be assigned _FloorSurface_ semantically to describe semantic information of indoor spaces and to avoid semantic conflicts. The same is true for the simplification of ceilings and roofs, that is, center surfaces are used, and the simplified surfaces can be assigned _CeilingSurface_ semantically. The layouts of rooms on different stories are not vertically identical.