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

3.3 Angle Measurement System 43vertically. Thus, the vertical positions are guaranteed with a sufficient accuracy (< 0.5 mm).sphere 1 sphere 2 sphere 3Cm) (u) (m)XJaser scanner,''sphere 4 fa*) 3Er () sP^ere 6sphere 7() () ()sphere 8 sphere 9Figure 3.21: Experimental setup: test field of observation pillars for determining the accuracy of the angle mea¬surement system. The laser scanner is set up on the center pillar and the spheres on the surrounding pillars (topview).The spheres were scanned in all provided scanning modes varying from 'superhigh' to 'preview'in twofaces.Cartesian coordinates of the center points were calculated and the sphericalcoordinates were de¬rived. Spherical coordinates obtained by the measurement in two faces were averaged to eliminate typicalinstrumental errors, e.g.error of collimation axis, error of horizontal axis, i.e. tiltingaxis. Based on the re¬sulting horizontal and vertical angles, the angle differences between two adjacent pillars can be computed,in horizontal and vertical directions. Overall, the laser scanner positioned on the center pillarby eight other pillars. Selecting one pillar as a reference, seven angleand the others can be derived.is surroundeddifferences between the selected oneThus, there are eight possibilities to choose from for the reference pillar.Overall, 7x8 angle differences in horizontal and in vertical direction can be used to calculate a mean valueand the precision of the mean value to asses the accuracy of the angle measurement system.Calibration Track LineThe second method uses the calibration track line, which has a well-known trajectory,cf. Section 3.1.1.The trolley including a sphere was positioned along the track line in specific positions and the sphere wasscanned from the pillar in extension with the track line (pillar number 2000).the spheres were derived according to the local scanner system.Then, the center points ofA coordinate transformation10 can beperformed to compare the derived center points of each sphere with its nominal position with respect tothe reference system of the calibration track line, cf. Figure 3.4.as follows:The resulting residuals can be interpreted• Residuals along the track line (x-direction): accuracy of the distance measurement system• Residuals in transverse direction of the track line (y-direction): accuracy of the angle measurement systemof the horizontal direction• Residuals in vertical direction of the track line (z-direction): accuracy of the angle measurement system ofthe vertical directionThe residuals, resulting from the transformation of coordinates, are expressedin metric values. The conver¬sion of the metric values of the residuals, expressed in millimeter, to angles, expressed in degrees,is crucial10The transformation can be done either as a complete 3D-transformation or as a separate 2D-transformation containing the xy-, thexz- and the yz-direction. The results of the residuals are identical, but the transformation parametersare different.