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

62 3. Calibration of Terrestrial Laser ScannerTable 3.13:Results of the adjusting hyperbola concerning the error of the collimation axis in horizontal direction.Parameter a defines for the gradient and parameter b defines the limit.Data Series a[°/m] b[°] aa[°/m] *fe[°]sphere 12 cm (1) 0.064 -0.016 0.0025 0.0007sphere 12 cm (2) 0.049 -0.016 0.0022 0.0006sphere 15 cm (1) 0.046 -0.014 0.0007 0.0002sphere 15 cm (2) 0.038 -0.014 0.0034 0.0010with the ranges and the difference tp of the horizontal directions in two faces corrected by the twofoldcollimation error. The resulting values for the eccentricity are summarized in Figure 3.35. The eccentricitye can be described by a mean value of 0.9 mm and a precision of 0.1 mm. The mean value is based on dataup to 15 m because the influence of the eccentricity decreases and then reaches the same degree of precisionas the precision of the angle measurement system, cf. Table 3.12.2.0eccentricity (horizontal component)>| 0.0co>uo4> -10— 12cm (1)—15cm (1)—12cm (2)—15cm (2)"200 2 4 6 8 10 12 14 16 18 20range [m]Figure 3.35: Eccentricity of the collimation axis in horizontal direction. The eccentricity of the collimation axis inthe horizontal direction is derived by using spheres with the diameters of'12 cm and 15 cm based on two independentsetups (1) and (2).In summary, the error budget of the collimation axis, consisting of the collimation error and the eccentricity,was derived. Considering the influence of both errors shown in Figure 3.34, it can be seen that the adjustinghyperbolaeintersect the abscissa. This means at the intersection point, about 3 m to 4 m), the error of thecollimation axis has no influence on the horizontal direction. The interpretationis the collimation error andthe eccentricity have opposite signs and they compensate each other. However, the most important aspectis that the influence of the error of the collimation axis is not constant for close ranges. The correction of thehorizontal angle data has to be completed separately according to the range and its error budget.Vertical ComponentSecond, the component in vertical direction is discussed. The results obtained by the two independentsetups are shown in Figure 3.36. It can be seen that the results of each setup obtained by the two sphereswith different diameters match each other.However, the results of the two independent setupseach other. With the exception of the first two meters, no systematic effects, depending on the range,do not fitcan beseen. The lines approximately describe a constant line. Thus, a significant influence of an eccentricity of thecollimation axis in the vertical direction cannot be seen. The constant line defines a collimation error in thevertical direction, which can be interpreted as an index error in the vertical encoder. Further assumptionsof the factors that influence the orientation of the laser beam concern an offset between the outgoing laserbeam and the encoder readings,variations on the rotation time and wobble effects of the horizontal axis.