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

-> CityGML is an open conceptual data model for the storage and exchange of virtual 3D city models. It is defined through a Unified Modeling Language (UML) object model. This UML model extends the ISO Technical Committee 211 (TC211) conceptual model standards for spatial and temporal data. Building on the ISO foundation assures that the man-made features described in the City Models share the same spatial-temporal universe as the surrounding countryside within which they reside. The aim of the development of CityGML is to reach a common definition of the basic entities, attributes, and relations of a 3D city model. This is especially important with respect to the cost-effective sustainable maintenance of 3D city models, allowing the reuse of the same data in different application fields.<sup>1</sup>
+> CityGML is an open conceptual data model for the storage and exchange of virtual 3D city models. It is defined through a Unified Modeling Language (UML) object model. This UML model extends the ISO Technical Committee 211 (TC211) conceptual model standards for spatial and temporal data. Building on the ISO foundation assures that the man-made features described in the City Models share the same spatial-temporal universe as the surrounding countryside within which they reside. The aim of the development of CityGML is to reach a common definition of the basic entities, attributes, and relations of a 3D city model. This is especially important with respect to the cost-effective sustainable maintenance of 3D city models, allowing the reuse of the same data in different application fields.[^1]
 
 In order to describe a city model, CityGML utilizes thematic modules. The base module is the CityGML Core which gets extended by defined thematic modules such as Building, Bridge, Vegetation, Transportation, WaterBody, Relief, LandUse, Appearance etc. In this tutorial, the building module will be used to explain the CityGML concepts. Examples will be given in CityGML 2.0 as most existing tools are using this version. However, the examples will be given in CityGML 3.0 XML encoding as well to see the differences of CityGML 2.0 and CityGML 3.0.
 
@@ -68,7 +68,7 @@ L. Rothengass: Generierung und Evaluierung dreidimensionaler Landschaftsmodelle
 
 ### CityGML Version 3.0
 
-1. [OGC City Geography Markup Language (CityGML) 3.0 Conceptual Model Users Guide, OGC document 20-066](http://www.opengis.net/doc/UG/CityGML-user-guide/3.0)
+[^1]: [OGC City Geography Markup Language (CityGML) 3.0 Conceptual Model Users Guide, OGC document 20-066](http://www.opengis.net/doc/UG/CityGML-user-guide/3.0)
 
 [OGC City Geography Markup Language (CityGML) Part 1: Conceptual Model Standard, OGC document 20-010](https://docs.ogc.org/is/20-010/20-010.html)