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

!36 3. Calibration of Terrestrial Laser Scanner10—90% (white)-—60% (grey)20% (dark grey)distance precision^*/r"'9. 4-^:i- ^i--4-*"^.,>'" /K-_ji^ -zSy „*•""* """ , ,10 15 20 25 30range [m]35 40 45 50Figure 3.13: Precision of the distance measurement system based on the 'static mode'. The targets used have differentvalues for reflectivity, namely 90% (white), 60% (grey) and 20% (dark grey).laser scannerspihere (interferometertrolleyrri i't a * i i | * s tri i til i i a t'i * 3 t j 'r i i * t * r r 'i j i j r r t s a * n i j; distance (tnteiferometer)distance (laser stanr«)calibrated fli stanceFigure 3.14: Calibration setup for the distance measurement system in the 'scanning mode' of the laser scanner usingspheres.The calibration procedure is performed in all provided scanning modes: 'superhigh', 'high', 'middle', and'preview'. These modes differ in the resolution of the point cloud since the point-spacing in the horizontaland vertical directions decrease from 'superhigh' to 'preview'. For the scanning process, the operator canchoose additional settings to modify the range mode6 and the noise7. The area containing the spherescanned is selected manually based on a preview scan acquired by the lowest resolution, which is madewithin a short time. The laser scanner was set up on the observation pillar, which is located in extensionwith the track line (pillar number 2000, cf. Figure 3.4). The spheres were positioned by the trolley alongthe calibration track line and the nominal distances were provided by the interferometer to high degree ofaccuracy, cf. Figure 3.14. The results obtained by this investigation helpsystem regarding:to beto assess the distance measurement6Referring to Section 3.2 one can choose between the range modes, 'close' and 'far'.What has been recommended is the rangemode, 'far', since this mode operates with the full laser power. On the contrary, the mode 'close' only uses half of the laser power7The noise differs between 'default' and 'low'. In the operating mode 'low', the scan time increases and the noise decreasesin accordance with the manufacturers' specifications.In the investigation procedures, no significant improvement concerning thequality of the point clouds have been seen.