# An Overview of Indoor Positioning and Mapping Technology Standards
## 2 Related Work
### Standards for Indoor Positioning, Mapping and Navigation
#### 2.3.3 OGC CityGML and IndoorGML  
In the last decades, 3D city models have been used in dozens of application domains for diverse purposes, such as urban planning, facility management, 3D cadaver, energy demand estimation, aiding positioning and navigation, and emergency response, etc. [47]. CityGML is an open encoding standard of the Open Geospatial Consortium (OGC) for representation, storage, and exchange of semantic 3D city models based on the Geography Markup Language version 3.1.1 (GML3) [82]. Over a decade of development, CityGML is generally considered as the most internationally widespread open reference standard for 3D city model management, storage, and exchange, partly because of its characteristics, such as [82]:  
* Modularization The CityGML data model is thematically decomposed into a core module and thematic extension modules. The core module comprises the basic concepts and components of the CityGML data model, and based on the core module, each extension covers a specific thematic field of virtual 3D city models. CityGML 2.0 introduces 13 thematic extension modules, i.e., Appearance, Bridge, Building, CityFurniture, CityObjectGroup, Generics, LandUse, Relief, Transportation, Tunnel, Vegetation, WaterBody, and TexturedSurface. These modules contain one or more classes representing specific types of objects, and the most used module in practice is the Building module [83]. * Application Domain Extensions (ADE) Even though CityGML modularization provide a mechanism to extend core module, the advent of dozens of applications still requires some additional information that is not readily available in the CityGML data model, but it is beyond the scope of current standard version. Thereby, one of the significant requirements is to specify extensions to the existing CityGML data model. Basically, there are two ways to support augmenting the CityGML data model: through extending generic objects and attributes, and mainly using the ADE mechanism [84]. The ADE mechanism is orthogonally aligned with the modularization approach in CityGML, but it has to be defined in an extra XML schema definition file with its own namespace. Two examples for ADEs (i.e., Noise Immmission Simulation and Ubiquitous Network) are included in the CityGML 2.0 standard [82]. [PERSON] et al. [84] presented a survey of many more ADEs developed for a variety of purposes. * Multi-scale modelling CityGML supports five different levels of detail (LOD), that is, level LOD0 is essentially 2.5D Digital Terrain Model (DTM) over which an aerial image or a map may be draped; LOD1 is the well-known block model comprising prismatic buildings with flat roof structures; building in LOD2 has differentiated roof structures and thematically differentiated boundary surfaces; LOD3 denotes architectural models with detailed wall and roof structures potentially including doors and windows; while, LOD4 completes a LOD3 model by adding interior structures (for example, rooms, interior doors, stairs, and furniture) for buildings [82]. Each object in CityGML can have a different representation for every LOD, thus making possible efficient visualization and data analysis to meet different application requirements. * Other characteristics  
Other characteristics of CityGML are introduced in [82], including coherent semantical-geometrical modelling, closure surfaces, Terrain Intersection Curve (TIC), code lists for enumerative attributes, external references, city object groups, appearances, prototypic objects/scene graph concepts, and generic city objects and attributes. For example, semantical-geometrical modelling is one of the most important design principles for CityGML as real-world entities are represented by features with geometry and attributes, as well as relations and aggregation hierarchies between features.