Human operators use an intuitive procedure of rotating laser scanner point clouds into specific projections when trying to understand the structure of buildings contained in the data. This procedure has been formalized and incorporated into an automatic building model reconstruction procedure. The method derives the building orientation from the analysis of height histogram bins. Using the orientation, orthogonal 2D projections of the point clouds are generated, where roof planes occur as lines of points. These lines representing planes are extracted by a line tracking algorithm. In subsequent processing steps, the lines are extended to planes, and the planes are analysed for deviations from rectangular shape. Two or more neighbouring planes are grouped to generate 3D building models. Existing 2D GIS data may be used in the process to provide a reliable segmentation of airborne laser scanner datasets and to generate hypothesis supporting the actual building reconstruction. The method of 3D building model generation based on airborne laser scanner data and 2D GIS data has been tested on a dataset containing 250 buildings of different complexity. The success rate of correctly modelled buildings was close to 100% in simple regions with new-build residential buildings and dropped to below 50% in rather complex inner city areas or industrial areas.