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

@@ -4,6 +4,8 @@ In order to describe a city model, CityGML utilizes thematic modules. The base m
 
 ![CityGML 3.0 Module Overview](uploads/db6d3196812b0a7306be4ac5565e13f1/fig_3.png)
 
+CityGML Module Overview ([OGC 20-066](https://docs.opengeospatial.org/guides/20-066.html))
+
 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.
 
 ![CityGML-2](uploads/1d896b57e0343a215e1b404463334145/CityGML-2.png)