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

50 3. Calibration of Terrestrial Laser ScannerThus, the surveying has to be performed by the intersection from two viewpoints by using theodolites12The investigation was carried out in the test field of observation pillars The base of the two viewpointsused is well-known because the coordinates of the observation pillars were surveyed with high accuracyand thus, well-known, cfSection 313 The target mounted on top of the scanner was defined by a spike,representing a target, which is well-suited for angular measurementsThe laser scanner was rotated by discrete angles using the control software for the laser scanner13 Theangle increment for the °positions of the vertical axis was 45 Thus,surveyed Based on these positions, two questionscan be raisedeight positions for the spike were• Do the positions of the spike fit a circle7• Does the center point of the circle correspond to the nominal position of the observation pillar7The first question can be answered with an affirmative in that the positions of the spike do fit an adjustingcircle, cf Figure 3 26 Considering the residuals of the observations and the unknowns of the adjustment,including their accuracies, cf Table 3 8 and Table3 9, it can be concluded that the spike moved along acircle This means that the rotation is homogenous and the laser scanner was well-levelled A levellingerror caused by non-exact levelling produces an error in such a wayaxis does not take place in a horizontal plane but rather in an inclined planepositions of the spikes would yield to an ellipseinstead of a circlethat the rotation about the verticalThus, the projection of theAnother error that is also conceivableis one that can influence the rotation behaviour the wobble of the vertical axis, cf Section 3 4 2 However,such an error has to be rather large before it influences the horizontal positions of the spike (an inclinationof 4 °causes an error of about 0 1 mm)Table 3.8: Residuals of observationsData Point Residuals vx [mm] Residuals vy [mm]1 0 01 -0 012 0 03 0 063 -0 01 -0 104 0 01 -0 065 -0 02 0 036 -0 01 0 027 0 00 0 068 -0 01 0 06Table 3.9:Unknowns of the adjustment including their precisionsValue [m]Precision [mm]center point (real)x14 930380 02y7 807100 03radiusr0 042860 23center point (ideal)x14 9301801y7 8073601The second question can also be answered with an affirmative in that there was no significantoffset in thereal rotation center versus its ideal rotation center, cf Table 3 9 The offsets in the x- and y- directions have12The accuracyof distance measurements of total stations does not meet the required accuracyof tenths of a millimeter13The laser control software LRViewer and LRCServer by Zoller+Frohlch allows the rotation of the laser scanner m a specific direchon with a resolution of 0 01 °