AUTOMATIC EXTRACTION OF VERTICAL WALLS FROM MOBILE AND AIRBORNE LASER SCANNING DATA

摘要

Building outlines in cadastral maps are often created from different sources such as terrestrial surveying and photogrammetric analyses. In the latter case the position of the building wall cannot be estimated correctly if a roof overhang is present. This causes an inconsistent representation of the building outlines in cadastral map data. Laser scanning can be used to correct for such estimation inconsistencies and additional occurring changes in the building shape. Nowadays, airborne (ALS) and mobile laser scanning (MLS) data for overlapping areas are available. The object representation in ALS and MLS point clouds is rather different regarding point density, representation of object details (scale), and completeness, which is caused by the different platform position i.e. distance to the object and scan direction. These differences are analysed by developing a workflow for automatic extraction of vertical building walls from 3D laser scanning point clouds. A region growing segmentation using Hough transform derives the initial segments. These are then classified based on planarity, inclination, wall height and width. The planar position accuracy of corresponding walls and completeness of the automatically extracted vertical walls are investigated. If corresponding vertical wall segments are defined by a maximum distance of 0.1 m and maximum angle of 3 deg then 24 matches with a planimetric accuracy of 0.05 m RMS and 0.04 m standard deviation of the X- and Y-coordinates could be found. Finally the extracted walls are compared to building outlines of a cadastral map for map updating. The completeness of building walls in both ALS and MLS depends strongly on the relative position between sensor and object. A visibility analysis for the building facades is performed to estimate the potential completeness in the MLS data. Vertical walls in ALS data are represented as less detailed facades caused by lower point densities, which is enforced by large incidence angles. This can be compensated by the denser MLS data if the facade is covered by the survey.