The paper aims with some significative aspects of the classification, segmentation and 3D modelling procedures of laser data acquired over the city of Gorizia with an Optech ALTM 3033 system using the well known software TerraScan® of Terrasolid. After a preliminary classification of the points belonging or not to the terrain (filtering), a more detailed classification has permitted to correctly identify the buildings, the vegetation and other objects of small dimension. Within the zones classified as “building”, the irregular areas approximately corresponding to the various planes of the roof have been automatically delimited by 3D polilines (segmentation and vectorialization). Starting from these lines, the automatic extraction of the roof planes have been executed, properly structuring the topological relationships of the various planes and lines of the roof. In this way, the polyhedric model of the building (modelling) has been obtained, from which it is possible to automatically compute its volume