| ... | ... | @@ -22,28 +22,26 @@ The last level of detail, LOD4, now also models the interior of the building. At |
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Other [example data set](https://schemas.opengis.net/citygml/examples/2.0/building/) in CityGML 2.0 LoD0 to LoD4 is available at the OGC CityGML 2.0 standard repository.
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LOD1 and LOD2 are the most common level of details produced by users globally and used in different applications. For some applications, such as noise simulation, an abstract model or a low level of detail (LOD1) is often useful for the analysis and modelling of entire urban areas. In case of urban energy simulation in which individual buildings are simulated, LOD2 models are often used because of its geometrical detail of roof surfaces as well as the introduction of semantic properties. LOD1 and LOD2 models of various cities and districts are usually made available as [open data](https://transfer.hft-stuttgart.de/gitlab/coors/3d-stadtmodelle/-/wikis/DE/Open%20Data). In contrast, due to the high effort involved in creating LOD3 and LOD4 models are often not available for entire urban areas or at most they are available in small excerpts or individual building.
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LOD1 and LOD2 are the most common level of details produced by users globally and used in different applications. For some applications, such as noise simulation, an abstract model or a low level of detail (LOD1) is often useful for the analysis and modeling of entire urban areas. In case of urban energy simulation in which individual buildings are simulated, LOD2 models are often used because of its geometrical detail of roof surfaces as well as the introduction of semantic properties. LOD1 and LOD2 models of various cities and districts are usually made available as [open data](https://transfer.hft-stuttgart.de/gitlab/coors/3d-stadtmodelle/-/wikis/DE/Open%20Data). In contrast, due to the high effort involved in creating LOD3 and LOD4 models are often not available for entire urban areas or at most they are available in small excerpts or individual building.
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# **CityGML 2.0 Building Data Model**
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The building model defines the geometric and semantic properties of a building in CityGML. A building or a building part can be modeled as Solid (B-rep) or MultiSurface. The semantic objects for a BoundarySurface geometry in LOD2 are: RoofSurface, WallSurface, GroundSurface, and ClosureSurface.
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The CityGML building data model (shown below as a UML diagram) defines the geometric and semantic properties to represent a building object in its different LODs. A building object is either a building (Building) or a building part
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(BuildingPart). A building can consist of any number of building parts; these in turn can contain other building parts. In principle, a building object in CityGML can be broken down into many hierarchically linked building parts representing real world building construction. The common properties of both Building and BuildPart classes are gathered in the AbstractBuilding superclass, which can be modeled with MultiSurface or a Solid geometry. The semantic properties for a BoundarySurface modeled with a MultiSurface geometry are defined beginning from LOD2. For example, semantic properties of RoofSurface, WallSurface, GroundSurface, and ClosureSurface are added in LoD2. From LOD3, in addition to LoD2 the wall and roof surfaces can also have semantic properties of door and window openings. For LOD4, in addition to LOD3 the semantic properties of a Room bounded by FloorSurface, CeilingSurface, InteriorWallSurface can also be added.
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CityGML also defines attributes for every city object. The standard defines two types of attributes: Schema attributes and generic attributes. Schema attributes are part of the schema definition of each thematic module.
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The building schema defines the attributes: function, yearOfConstruction, yearOfDemolition, measuredHeight, storeysAboveGround etc. Generic attributes are defined by the user to store additional information in a city object. There is no restriction in the number or the names of the generic attributes as long as they conform to the XML syntactic rules.
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UML diagramm of Building Module
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The CityGML standard also defines attributes for every CityObject. The standard defines two types of attributes: Schema attributes and generic attributes. Schema attributes are part of the data model for each thematic module. The CityGML building data model defines schema attributes such as building class, function, yearOfConstruction, yearOfDemolition, measuredHeight, storeysAboveGround etc. Values of certain schema attributes for example building class, function, usage, roof types must be defined as a code of a valid XML code list. A sample code list of all the schema attributes which must be defined as a code is available [here](https://www.sig3d.org/codelists/citygml/2.0/building/2.0/). Custom or external code list based on individual study areas can also be developed and used. Generic attributes are defined by the user to store additional information in a city object. There is no restriction in the number or the names of the generic attributes, as long as they conform to the XML syntactic rules.
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example: HFT (one building) (figure from Athanasios) 
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Further info on the CityGML building data model, its XML schema definition (XSD) and conformance requirements are available in the [CityGML 2.0 standard document](https://portal.ogc.org/files/?artifact_id=47842) Ch. 10 section 10.3.
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XML encoding of the CityGML building data model can be found [here](https://schemas.opengis.net/citygml/building/2.0/building.xsd) as a XSD document.
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link to open access data set of HFT campus
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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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https://transfer.hft-stuttgart.de/gitlab/neqmodplus/neqmodplus-steinbeis/-/wikis/NeqModPlus/NeqModPlus/HfT_Campus_CityGML_2.0_Models
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(uploads/d19828ed823eb14763b58d54cd8388e8/image.png)
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**HFT Stuttgart buildings modeled in LOD 1,2,3 and 4**
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# **New is CityGML 3.0 Building Data Model**
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