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

26 3. Calibration of Terrestrial Laser Scanner3.1 Laboratories and Tools for CalibrationThe IGP is equipped with various kinds of laboratory facilities, test fields, etc. Thus, geodetic instrumentscan be calibrated regarding different aspects.In the following, some laboratory facilities and test fields,which play an important role in the calibration of the terrestrial laser scanner, are introduced.3.1.1 Calibration Track LineThe calibration track line at the IGP has a length of roughly 52 m. A trolley moves on the track line, eitherautomatically or manually, cf. Figure 3.2. This trolley is tracked by a laser metric system (HP 5529A dy¬namic calibrator), which is based on interferometry. Thus, the position of the test trolleyon this track linecan be located within a high accuracy of less than 0 1 mm1. Atmospheric conditions , i.e. air temperature,air pressure and air humidity, are also collected as they are required for atmosphericmeasured by the interferometer to guarantee high accuracy.Therefore, lab conditions aretemperaturestabilizedby air conditioning within a temperature rangecorrection of distancesof 20 °C ± 0 5°C.The distance measurementsystem of geodetic instruments can be calibrated by comparing the measured distances acquired by theinstrument to be calibrated with the nominal distances provided bythe interferometer.Therefore, the in¬strument to be calibrated is set up at the beginning of the track line in a way that the instrument is alignedin extension with the track line.An observation pillar is installed and facilitates the required setup easily.On the test trolley, a target can be mounted to measure the distances between the test trolleyand the instru¬ment. The additive constant between the mechanical zero point of the track line and the observation pillaris calibrated and well-known. Thus, relative as well as absolute distance measurements can be performed.Figure 3.2: Calibration track line with a length of52 m including the test trolley movingon the track line.The 3D trajectory of the calibration track line is checked periodically. The optimal track can be approx¬imated by a line that should run straight horizontally and vertically.Since an optimal straightline canrarely be established, the real run of the track line has to be identified. Generally, three different parametersare required: the straightness of alignment, the verticality of alignment and the cant, i.e.ent. The trajectory of the calibration track line regarding these parameters are shown in Figureupper figure, the straightness of alignment and the verticality of alignmentcan be seen.transverse gradi¬3.3. In theThe variation inthe vertical direction shows a maximum relative value of 1 mm. According to Abbes comparator principle,the laser beam of the interferometer and the measurement beam of the instrument to be checked has tobe in a straight line.Since this constraint can rarely be met with the designthe verticality of alignment is of great importance. But the maximum changeof the calibration track linein the vertical direction can1This accuracy represents for the resulting accuracy of the mechanical zero point of the calibration track line, the orientation of thetest trolley and the accuracy of the laser interferometer The accuracy of the HP interferometer is given by 0 2 /im + 05 10 ~6 D,where D is the distance m [m], the resolution is given by 0 1 /im