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Incidence angle influence on the quality of terrestrial laser scanning points

A terrestrial laser scanner measures the distance to an object with a precision in the order of millimeters. The quality of each single point in a point cloud affects post-processing applications, such as deformation analysis or 3D modeling. The quality of a scan point is influenced by four major factors: instrument calibration, atmospheric conditions, object properties and scan geometry. In this paper, the latter factor is investigated focusing on the influence of incidence angle, i.e. the angle between incoming laser beam and surface normal, on the precision of a scan point. It is shown that by considering the influence of incidence angle on the signal to noise ratio, the increase in measurement noise with increasing incidence angle can be successfully modeled. The implications of this model are demonstrated on two practical experiments. In the first experiment, a reference plate is scanned at a fixed distance but under different scan angles. The analysis shows that also in a practical setting the influence of incidence angle could be successfully isolated, allowing the conclusion that above 60o the incidence angle dominates the scan point precision. In the second experiment it is demonstrated that for a typical point cloud of a room, 20% of the measurement noise is due to incidence angle. The results of this research make it feasible to optimize the scan locations in a measurement setup in the sense that noise due to incidence angle is minimized.