EN/CityGML-Tutorial/chapter-2:-Building.md

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 In CityGML, a building object can be defined in different Level Of Details (LOD). For example, in CityGML 2.0, a building can be defined as LoD 0,1,2,3 and 4. With an increasing level of detail, optional components such as geometric details, interior or exterior fittings, door and window openings, and interior spaces and their fittings are added. The same concept also applies to the other modules (CityObjects) of CityGML.
 
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 ![Abb-4.5](uploads/629dfa12271f64e43e1ee6d9099e810b/Abb-4.5.png)
 
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 **A simple building represented in LOD0 (Left) to LOD4 (Right)**
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 Example CityGML 2.0 building model of HFT Stuttgart modeled in LOD 1,2 and 3 can be downloaded from [here](https://transfer.hft-stuttgart.de/gitlab/neqmodplus/neqmodplus-steinbeis/-/wikis/NeqModPlus/NeqModPlus/HfT_Campus_CityGML_2.0_Models). Due to privacy reasons, LOD4 model of HFT Stuttgart buildings are not shared publicly.
 
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 ![image](uploads/be5b68456cd08ecca62b5da0f61809db/image.png)
 
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 **HFT Stuttgart buildings modeled in LOD 1,2,3 and 4**
 
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 # **New in CityGML 3.0 Building Data Model**
 
 CityGML 3.0 revises the LOD concept, which now allows representing the interior of a building and other CityObjects at any level of detail. LOD4, which is used for representing the interior of objects in CityGML 2.0 is removed and only the LODs 0/1/2/3 remains. The interior of objects can now be integrated with the LODs 0/1/2/3. It is also possible even to model the outside shell of a building in LOD1, while the interiors in LOD2 or 3. 
 
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 ![CityGML3.0_LOD](uploads/0286c8969f6a3c1efbde50bc6c3f89a2/CityGML3.0_LOD.png)
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 **Representation of CityGML 3.0 Building in LOD 0 (Left) and LOD2 (Right) with interiors**[^1]
 
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 CityGML 3.0 also revising the data model of Building modules. New super(abstract) classes are introduced to inprove interoperability with other standardized data models such as IFC. First _AbstractConstruction_ class as subclass of _AbstractOccupiedSpace_ is introduced which is associated with the different thematic surfaces (previously boundary surfaces in CityGML 2.0). CityObjects such as buildings, bridges, and tunnels, are defined as subclasses of the class _AbstractConstruction_. Furthermore, a new feature type AbstractConstructiveElement is introduced and corresponding subclasses for example BuildingConstructiveElement is defined to allow easy mapping of constructive ements such as IfcWall, IfcBeam, IfcSlab etc. from BIM data sets modeled in IFC standard. Another new class _AbstractBuildingSubdivision_ which is modeled as a subclass of _AbstractLogicalSpace_ is introduced to model BuildingUnit and Storey for representing real world building units (e.g apartments) and building storeys. Class _AbstractBoundarySurface_ from CityGML 2.0 is now revised as _AbstractThematicSurface_ have two subclass of AbstractConstruction and FillingSurface with a property of boundary. Doors and Windows which were elements of AbstractOpening class in CityGML 2.0 are represented as filling elements in CityGML 3.0 in addition to the classes WindowSurface and DoorSurface introduced to represent filling surfaces. Similarly GroundSurface, RoofSurface, WallSurface, ClosureSurface etc. are modeled as ConstructionSurface in CityGML 3.0. A simplified excerpt of CityGML 3.0 building data model is shown below as a UML diagram.
 
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 ![CityGML_3.0_Building_UML.drawio](uploads/a0c9c7a0dd9d71b968ec6ea2d73187df/CityGML_3.0_Building_UML.drawio.png)
 
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 As mentioned before, CityGML 3.0 is a recent development and thus has a limited support with exisitng softwares. A free tool to convert CitGML 2.0 datasets to CityGML 3.0 is available [here](https://github.com/tum-gis/citygml2-to-citygml3) as an open source release. Converted CityGML 3.0 datasets of building and other CityObjects from CityGML 2.0 is available at the [OGC CityGML 3.0 standard repository](https://github.com/opengeospatial/CityGML3.0-GML-Encoding/tree/main/resources/examples).
 
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