Large area covering LiDAR height models – problems and solution

The company BIMTAS, belonging to the Greater Istanbul Municipality, generates a digital height model (DHM) for the area of the Greater Istanbul Municipality with a Riegl LSM-Q680i full waveform laser scanner with 400kHz and a used field of view of +/-30°.
A precise absolute orientation requires ground control points (GCP), presented by height values of gable roofs to allow also an improvement of horizontal shifts. Because of the very high number of flight lines it is not possible to have GCP for any flight line, so sub-blocks of approximately 10 flight lines have been combined by 3D-transformation based on height points on planes. These sub-blocks have been transformed to the GCP. The varying overlap of neighbored LiDAR-strips in the range of 40% allows a three-dimensional transformation of neighbored, but if 10 strips of a sub-block are transformed together this may lead to unfavorable error propagation. The sub-blocks are only shifted to control point groups with points on inclined roofs and on the ground. Nevertheless a check of the transformed LiDAR-strips by check points results in a vertical accuracy of better as 5cm. Another possibility of joining overlapping LiDAR-strips is a filtering of the point cloud, eliminating points on vegetation and at building limits. Such a transformation uses quite more corresponding points as the method based just on planes and is robust also in areas only with few and not well distributed buildings. The result of transformation is on a similar level.
In addition to the original digital surface models (DSM) also DHM with the points of the bare ground have been generated.
The major problem of the LiDAR-points is the data amount. With a density of 16 points/m² a single strips has approximately 500 million ground points, exceeding the capacity of several programs.