# A Simplified CityGML-Based 3D Indoor Space Model for Indoor Applications
## 5 Conclusions and Future Work  
As the basis for indoor navigation and emergency rescue, a 3D indoor model should be able to provide a variety of indoor spatial information, such as indoor locations, building components, and indoor spatial adjacency relationships. A simplified 3D indoor model that contains accurate indoor spatial geometry information, rich semantic information, and spatial topological relationship information is proposed in this paper to achieve efficient indoor navigation and rapid emergency rescue. The simplified 3D indoor model in this research has the following advantages. Flexible LOD description for the building interior. Considering that the LODs concept of CityGML2.0 can no longer meet the requirements of emerging indoor applications, the model draws on the LODs concept of CityGML3.0, describing the indoor LOD of buildings in detail, and ignoring the outdoor LOD. The model eliminates the exterior building information that is not of interest to indoor applications, avoids data redundancy, and provides basic data for indoor applications. Lightweight description for indoor spaces. The building components are simplified in the model, and the geometric properties of the elements are preserved as much as possible. Although the location of the building elements may change after simplification (e.g., replacing the original wall with a center surface), the centered element can represent the original component without loss of accuracy for indoor navigation. Moreover, the original semantic properties of building components are retained, and appropriate semantic properties are assigned to describe the indoor space characteristics, avoiding  
Figure 26: Excerpt from the CityGML dataset of an interior wall shared by two adjacent rooms. (**a**) XML description of an interior wall “wall_2_level_2”. (**b**) The wall “wall_2_level_2” is referenced by a room “room_second_1” through XLinks. (**c**) The wall “wall_2_level_2” is referenced by a room “room_second_2” through XLinks. semantic inconsistency after simplification. Third, the model is beneficial as it describes the adjacencies of indoor spaces. For example, replacing the original wall with a center surface allows to represent the topological relationship of indoor spaces using XLinks, providing basic data for indoor spatial query and path generation. Facilitating data interoperability. The geometric, semantic, and topological properties of the model follow the CityGML schema, which is based on GML. GML provides a standardized geometry and topology model of geographical features, making it easy to combine CityGML with other geospatial data to facilitate interoperable data exchange. However, several aspects still need to be investigated further and solved. First, the model provides a simplified description for complex building components and indoor spaces, but does not consider interior facilities, such as furniture. In reality, various facilities and furniture, both movable and nonmovable, are placed in indoor spaces. These furnishings occupy a certain amount of indoor space that is inaccessible, and the available unoccupied space needs to be calculated for indoor navigation and emergency rescue. CityGML 2.0 LOD4 is able to geometrically and semantically describe furniture; however, as mentioned above, all detailed indoor information, including furniture, is represented in LOD4. Additionally, LOD4 describes the location, shape and size of furniture in detail, which is too elaborate for indoor navigation, resulting in data redundancy. Furthermore, some furniture, such as chairs, can move around and thus the unoccupied space will change at any time. Accurately and appropriately describing furniture without data redundancy and representing the mobility of furniture is a major challenge in building 3D indoor models. Second, the model describes the topological adjacencies of indoor spaces through CityGML XLinks to facilitate the generation of indoor space paths. However, detailed indoor path information has not been provided in the model.