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234 Journal of the Spanish Institute for Strategic Studies Núm. 11 / 2018 procedure is only valid if the point clouds have similar geometries and they are partially aligned51. Finally, and as a result of the combination of the different point clouds, a single (and complete) point cloud of the whole area is obtained, which will be used in later steps for 3D modelling (Figure 5.b). 3D Modelling and texturisation 3D modelling procedures in Cyclone provides a simplified reconstruction of each of the buildings. In order to homogenise criteria and requirements, the Open Geospatial Consortium (OGC) has defined its own data model called CityGML that defines five increasing levels of representation of 3D models ranging from LoD-0 to LoD-4 (both included) depending on the level of detail represented52. The software programmes SketchUp and Autodesk 3ds Max, which features modelling tools for redefining and incorporating details, are used in the reconstruction and enhancement of the models obtained with Cyclone. Once the geometry of the models is defined, the next step is the incorporation of textures, for which both geometrically and radiometrically rectified images are used. Any image editing software, such as Photoshop, allows one to select the area of interest of the photographs with respect to each of the defined geometric principles, in addition to rectifying the geometry and adjusting the radiometry of the parameters and/or elements of interest. Once the textures are prepared, they are carefully pasted and adjusted on each of the faces of the CAD model, and finally a photorealistic 3D model of each of the buildings is obtained. Image 6: (a-b) process of CAD modelling from point clouds 51  HAN, Lina; CHONG, Yanwen; LI, Yuanting and FRITSCH, Dieter: «3D Reconstruction by combining terrestrial laser scanner data and photogrammetric images». In: Proceedings of the Asia Assoc. of Remote Sensing, 2014, Nay Pyi Taw, Myanmar. 52  KOLBE, Thomas H.; GRÖGER, Gerhard and PLÜMER, Lutz: «CityGML - Interoperable access to 3D city models», in: International Symposium on Geo-Information for Disaster Management GI4DM, 2005, Delft, Netherlands, pp. 21-23. http://revista.ieee.es

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