Urban environments of modern cities are described digitally in large public databases and datasets of e.g. laser scanning and ortho photos. These data sets are not necessarily linked to each other, except trough their geometry attributes (coordinates), which are mutually displaced and have a low degree of details. However, it is possible to create virtual 3D models of buildings, by processing these data. Roof polygons are generated using airborne laser scanning of 1x1 meter grid and ground plans (footprints) extracted from technical feature maps. An effective algorithm is used for fixing the mutual displacement of these datasets. The well known Hough Transform is extended to 3D for extracting planes from the point cloud. Generally speaking, planes are rejected if the clusters of points on these planes, do not span a considerable area. Furthermore, it is assumed that valid planes are close to parallel to one of the ground plan lines. Points corresponding to each valid plane are subtracted from the original point cloud, and the Hough Transform is performed on the remainder. By this approach, the disturbing influence of already evaluated points, is avoided. Small variations of the roof surface might lead to multiple slightly differing planes. Such planes are detected and merged. Intersecting planes are identified, and a polygon mesh of the roof is constructed. Due to the low precision of the laser scanning, a rule-based postprocessing of the roof is applied before adding the walls.
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