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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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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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- A CRS should not change through out the CityGML dataset,
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- A CRS should be defined once within <gml:Envelope>. For example:
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```plaintext
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<gml:boundedBy>
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<gml:Envelope srsDimension="3" srsName="urn:adv:crs:ETRS89_UTM32*DE_DHHN92_NH">
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<gml:lowerCorner>512665.636 5403023.0 257.0</gml:lowerCorner>
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<gml:upperCorner>512739.215 5403100.951 276.0</gml:upperCorner>
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</gml:Envelope>
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</gml:boundedBy>
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```
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**Or**
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```plaintext
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<gml:boundedBy>
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<gml:Envelope srsName="EPSG:5555" srsDimension="3">
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<gml:lowerCorner>512665.636 5403023.0 257.0</gml:lowerCorner>
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<gml:upperCorner>512739.215 5403100.951 276.0</gml:upperCorner>
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</gml:Envelope>
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</gml:boundedBy>
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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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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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