Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

90 5. Kinematic Laser ScanningThe accuracy of the orientation depends on the accuracy of the determination of the azimuthal direction tothe sphere and the precision of the horizontal encoder system.If the scanning profilesare not normal tothe moving direction, the misalignment can be corrected mathematically by using control pointsprofiles are shifted by a constant angle.This lever arm effect was discussed earlier.since theazimuthal direction(scanning profiles )A1S |azimuthal direction / ^^^HI(sphere centre) /oicalibration track linemoving directioninital director(azimuth),test trolley(laser scanner)Figure 5.2: Azimuthal orientation of the laser scanner. The azimuthal direction to the sphereazimuthal direction for the profiles can be determined by adding a specific angle (here: 90 °).can be derived and the5.1.2 Absolute Position and OrientationThe absolute position and orientation of an object is defined by three coordinates and three attitude angles.To measure the six degrees of freedom, different sensors and instruments can be used, e.g.acceleratorsensors, inclination sensors, tracking total station, global positioning system.Inertial navigation systems(INS) directly provide the desired six parameters.The sensors and instruments that should be used and areappropriate for the specific application depend on the desired accuracy and the environment. If the objectis in motion, these six parameters are time-dependent and define the 3D-trajectory.The kinematic application performed on the calibration track line allows for some simplifications.Thetest trolley moving along the track line is forced to run on a fixed track. The trolleydirections. The trajectory of the calibration track line is well-known and alreadyThis trajectory can be described by variations in the straightness of the alignment, bycannot run in otherdiscussed in Section 3.1.1.variations in thevertical alignment and by variations in the cant. The verification of the deviations of the different linearitiesshows only small variations. The measurement of the three attitude angles (to, ,trajectory is well-defined. For simplifications, it is also possible to approximate the trajectoryk) is not necessary since theas a constantline, which runs horizontally. The error budget of this approximation is in the same order of magnitude asthe accuracy of the point cloud acquired bythe laser scanner.The absolute position of the test trolley moving along the track line is measured bytion. The laser scanner is equipped with a prism holder to attach a prism, which is tracked byan automated total sta¬the total sta¬tion. Since in kinematic applications the synchronization between the distance and the angle measurementsystem of tracking total stations is of importance [Stempfhuber, 2004], asetup for the kinematic applicationis chosen so that the error resulting from a conceivable erroneous synchronization is minimized. The totalstation is positioned in extension from the track line (pillar number 2000, cf. Figure 3.4). The test trolleymovesaway or towards the total station, resulting mostly in varying distances rather than varying angledirections. The setup can be seen in Figure 5.1.