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# **Level of Detail (LOD) Concept**
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## **Level of Detail (LOD) Concept**
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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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| ... | ... | @@ -28,7 +28,7 @@ Other [example data set](https://schemas.opengis.net/citygml/examples/2.0/buildi |
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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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# **Coordinate Reference System (CRS)**
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## **Coordinate Reference System (CRS)**
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To integrate different spatial datasets in a single CityGML document, a correct CRS is of utmost importance and a key requirement. CityGML inherits GML3’s spatial capabilities of handling Coordinate Reference Systems (CRS) which is the usual way of denoting the spatial reference in GML 3.1.1. It is strongly recommended that within CityGML:
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- A CRS should be defined as three dimensional (horizontal*vertical CRS or a compound CRS),
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| ... | ... | @@ -53,7 +53,7 @@ To integrate different spatial datasets in a single CityGML document, a correct |
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```
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[epsg.io](https://epsg.io/) is a free tool that can help to find worlwide CRS.
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# **CityGML 2.0 Building Data Model**
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## **CityGML 2.0 Building Data Model**
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The CityGML building data model (simplified excerpt 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 with a property of boundedBy. 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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| ... | ... | @@ -84,7 +84,7 @@ Example CityGML 2.0 building model of HFT Stuttgart modeled in LOD 1,2 and 3 can |
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</div>
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# **New in CityGML 3.0 Building Data Model**
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## **New in CityGML 3.0 Building Data Model**
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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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| ... | ... | @@ -122,4 +122,6 @@ As mentioned before, CityGML 3.0 is a recent development and thus has a limited |
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</div>
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## **References**
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[^1]:[Löwner, M.-O., Gröger, G., Benner, J., Biljecki, F. and Nagel, C. Proposal for a new LOD and Multi-Representation Concept for CityGML. ISPRS Ann. Photogramm. Remote Sens. Spatial Inf. Sci (2016), IV(2/W1), 3–12.](https://doi.org/10.5194/isprs-annals-IV-2-W1-3-2016) |
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