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

44 3. Calibration of Terrestrial Laser Scannerbecause of the short ranges between 1 m and 20 m due to the limited range to scan spheres, cf. Section 3.2.2.Disregarding the first three meters and averaging the angular values, a mean value and the precision of themean value to assess the accuracy of the angle measurement system can also be providedin the horizontaland vertical directions. However, the linear alignment of the calibration track line is not well-suited fora 3D coordinate transformation, but provides an independent method for the investigation of the anglemeasurement system.Thus, the mean values obtained by the two different experimental setups can be compared. The specifica¬tions of the manufacturer state that the accuracy for the angle measurement system of both the horizontaland the vertical encoders are within 0.02 °(RMS).3.3.1 Horizontal EncoderThe precision of the horizontal encoder of the angle measurement system is shown in Table 3.3. The preci¬sion of a single measurement (ss) and the precision of the mean value (sm) are also given. The precision ofa single measurement shows good values for all scanning modes and can be assessed as sufficiently high.These values were derived by ten repeated measurements within the same experimental setup. The rangeto the sphere was approximately 3 m. The decreasing precision from the scanning mode 'superhigh' to'middle' conforms with an increasing range to the sphere in the scanning mode 'superhigh', i.e. the preci¬sion decreases with increasing ranges. The quality and the quantity of the point cloud worsen with eitherthe rangeor the scanning resolution.Table 3.3:Precision for the horizontal encoder of the angle measurement system gained by repeated scanning of asphere for the scanning modes 'superhigh', 'high'and 'middle'.Scanning Mode SS[°] sm[°]superhigh 0.001 0.0004high 0.002 0.0007middle 0.006 0.0020The accuracy of the horizontal angle measurement system obtained by the method usingobservation pillars are summarized in Table 3.4. The mean value as well as precisionbased on all 7 x 8 combinations of the horizontal angle between two pillars.values of the accuracy for the scanning modes 'superhigh', 'high'the results are reasonably.and 'middle'. Thethe test field ofof the man values areThe table shows equivalentprecision shows thatTable 3.4:Accuracy for the horizontal encoder of the angle measurement system obtained by using the test field ofobservation pillars.Scanning Mode Mean Value [°] sm[°]superhigh 0.011 0.002high 0.011 0.002middle 0.008 0.004The residuals transversal to the calibration track line representing the y-axis for assessing the accuracy ofthe angle measurement system are shown in Figure 3.22.In the upper figure, the residuals resulting fromthe coordinate transformation of the local scannersystem to the calibration track line system are plottedbased on the scanning modes 'superhigh', 'high' and 'middle'. The sphereThe lower figure shows the absolute values of the residuals expressed in degrees.all scanning modes nearly the same results can be achieved. The residuals are oscillatingused has a diameter of 15 cm.It can be seen that inbetween +2 mm