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

46 3. Calibration of Terrestrial Laser ScannerTable 3.5:Precision for the vertical encoder of the angle measurement system obtained by repeated scanning of asphere for the scanning modes 'superhigh', 'high'and 'middle'.Scanning Mode SS[°] sm[°]superhigh 0.001 0.0003high 0.003 0.0011middle 0.005 0.0017The accuracies of the vertical angle measurement system gained by the method using the test field of obser¬vation pillars are summarized in Table 3.6. The mean value as well as the precisionscombinations for defining a vertical angle between two pillars.accuracy of the scanning modes 'superhigh', 'high' and 'middle'. The precisionreasonable.are based on all 7 x 8The table shows equivalent values for theshows that the results areTable 3.6:Accuracy for the vertical encoder of the angle measurement system obtained by using the test field ofobservation pillars.Scanning Mode Mean Value [°] sm[°]superhigh 0.008 0.004high 0.010 0.002middle 0.014 0.005The residuals vertical to the calibration track line representing the z-axis for judging the accuracy of theangle measurement system are shown in Figure 3.23.In the upper figure the residuals resultingfrom thecoordinate transformation of the local scannersystem to the calibration track line system are plotted basedon the scanning modes 'superhigh', 'high' and 'middle'. The used sphere has a diameter of 15 cm. The lowerfigure shows the absolute values of the residuals expressed in degrees. It can be seen that in all scanningmodes, nearly the same results can be achieved. The residuals are oscillating between +2 mm and —2 mm,only in the scanning mode 'middle' do the residuals increase from ranges of more than 12 m. Consideringthe residuals in terms of degrees, it can be seen that the values do not exceed 0.01 ° with the exception of thefirst two meters, which are due to the unfavourable geometrical conditions for the translation from metricvalues to angularvalues in short distances.Generally, it can be summarized that the accuracy for the vertical encoder of the angle measurement systemlies within ±0.01 °.The results obtained by the two different experimental setupsfit each other and confirmthe achieved accuracy independently. Considering the residuals expressed in millimeters, it can be statedthat the accuracy of the vertical angle measurement system is not as crucial as the distance measurementsystem. Furthermore, the specifications of the manufacturer are in agreement. The precisionencoder is of a factor of ten better than the accuracy.for the verticalFurthermore, the angular precision and the angularaccuracy in the horizontal direction and in vertical direction are in the same order and fit each other.3.3.3 Angular ResolutionThe angle measurement system defines the vertical and horizontal resolution since the laser beam is de¬flected by the deflection unit in these two directions. Theinvestigation of the angularassess the manufacturer's specifications.The angular resolution is provided by using three different scanning modes: 'superhigh', 'high'dle'. A planar object aligned normal to the laser beam was scanned. For a specificresolution shalland 'mid¬distance a which isequal in the horizontal and vertical directions, the number of points in a horizontal and vertical profile