EN/CityGML-Tutorial/chapter-1:-Intro-CityGML.md

@@ -12,6 +12,34 @@ The following example shows a CityModel (CityGML 2.0) that makes use of several
 
 CityGML Model Stuttgart-Stöckach, the model itself was created by L. Rothengass as prt of his Master Thesis at HFT Stuttgart using the software [3dfier](https://github.com/tudelft3d/3dfier) and data provided by the State Office for Spatial Information and Land Development (LGL BW) (base DLM and LiDAR data) and the City of Stuttgart (tree cadastre and LoD2 building module). 
 
+## CityGML Core
+
+The following UML diagram defines the CityGML Core (CityGML 2.0).  
+![image](uploads/c0637b17c8b839570b05ca5f4f235a1c/image.png)
+
+A _City Model_ is a collection of _cityObjectMembers_. Each CityGML Module defines specific _CityObjects_ such as _Building_ objects, _Vegetation_ objects etc. Every _CityObject_ has a fixed set of specific attributes as specified in the UML model, and an _Appearance_ do define its visual properties. _GenericAttributes_ can be used to define additional attributes that are not explicitly specified in the UML model itself.
+
+The XML encoding of the UML diagram is defined in an XML schema. In CityGML version 2.0, the XML schema part of the CityGML standard document. An example of a typical XML document - the CityModel - is sketched here:  
+
+`<?xml version="1.0" encoding="UTF-8"?>
+<core:CityModel 
+           xmlns = … >
+  <core:cityObjectMember>
+      <bldg:Building> … </bldg:Building>
+      <bldg:Building> … </bldg:Building>
+      <bldg:Building> … </bldg:Building>
+      
+      <!-- Terrain -->
+      <dem:ReliefFeature> … </dem:ReliefFeature>
+      
+      <!– Lake -->
+      <wtr:WaterBody> … </wtr:WaterBody>
+      …
+  </core:cityObjectMember>
+</core:CityModel> ` 
+
+In CityGML 3.0, the UML model is defined in the conceptual model standard. Multiple encodings can exist to encode the conceptual model. One encoding is the XML encoding. With CityJSON, another encoding of the conceptual model is available. 
+
 
 ## References: 
 
@@ -25,6 +53,7 @@ CityGML Model Stuttgart-Stöckach, the model itself was created by L. Rothengass
 
 [Kutzner, T., Chaturvedi, K. & Kolbe, T.H. CityGML 3.0: New Functions Open Up New Applications. PFG 88, 43–61 (2020). https://doi.org/10.1007/s41064-020-00095-z](https://doi.org/10.1007/s41064-020-00095-z)
 
+
 ### CityGML Version 2.0
 
 [OGC City Geography Markup Language (CityGML) Encoding Standard, OGC document 12-019](https://portal.ogc.org/files/?artifact_id=47842)