# A Simplified CityGML-Based 3D Indoor Space Model for Indoor Applications
## 1 Introduction
However, the LODs concept in the current version is no longer sufficient to meet the requirements of emerging indoor applications, such as indoor navigation [25]. On the one hand, it is necessary to acquire the rooms adjacent to a certain room for indoor navigation. However, the descriptions for indoor spaces in LOD4 do not reflect two adjacent rooms sharing the same interior wall. On the other hand, LOD4 adds detailed indoor space information to LOD3, where the exterior shape of a building is the most detailed, which means that all the details of a building are described in LOD4. Indoor navigation requires access to detailed interior building information but not detailed exterior information. A revised LODs concept will be included in the forthcoming version: CityGML 3.0 [26]. The new definition of LODs is flexible to represent a detailed indoor model with a coarse exterior building shape, which may be beneficial to indoor applications, such as indoor navigation. Unlike several 3D modelling standards such as IFC and CityGML, which cover geometrics and semantics of interior spaces, IndoorGML [27] dedicates to model interior spaces for the purpose of indoor navigation. As an OGC standard, IndoorGML allows to represent and exchange indoor spatial information to construct indoor navigation systems. It provides a core data model to represent topological connectivity of indoor space and a network model in indoor space for indoor navigation. However, to avoid reduplicative efforts with IFC and CityGML, IndoorGML includes merely geometrics and semantics of building components, which could serve as a complementary model to IFC and CityGML for indoor navigation. Moreover, IndoorGML still has not yet represented the adjacency of interior spaces, such as two adjoining rooms sharing a wall, even though it describes the topological connectivity. With respect to 3D models for cadastral systems, the Land Administration Domain Model (LADM) [28] provides an abstract and conceptual model which describes basic administrative objects for land administration. The administrative units in LADM are described as legal spaces which can be connected with physical features in other models, such as the combination of LADM and CityGML to develop 3D cadastral systems [29; 30; 31] and the integration of LADM and IndoorGML to support indoor navigation [32] and emergency evacuation [33]. Since 3D cadastral objects are described as closed volumetric ones bounded by property boundaries, a large amount of literature focuses on the valid representation of 3D parcel of cadastral models and the automatic construction of 3D volumetric object [34; 35; 36]. However, 3D cadastral models rarely address the more detailed building components, such as doors and windows, or the topological adjacency relationship between rooms. Therefore, 3D cadastral models are not entirely suitable for describing 3D interior spaces. In this research, we introduce the simplified 3D indoor space model based on CityGML. The model describes accurate geometric information of building indoor spaces with rich semantic and corresponding topological information. The model is designed according to CityGML3.0's LODs concept, taking into account the requirements of indoor navigation and emergency rescue, highlighting the detailed description for indoor space information while ignoring exterior description of buildings. The geometric representation and semantic types of the model still follow the CityGML2.0 schema, and the topological relationship is described for adjacent rooms by XLinks. The remainder of this paper is organized as follows. In Section 2, we discuss the current CityGML LODs concept and simplified representation for buildings. The detailed design and representation of the simplified 3D indoor space model is proposed in Section 3. In Section 4, the CityGML dataset is extracted from BIM data using FME based on the model. Some conclusions and future work are given in Section 5.