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

42 3. Calibration of Terrestrial Laser Scannerin a maximum distance error of less than 0 2 mm Thus, the deviation of the modulation frequencyof thefine channel component to its nominal frequency does not influence the distance measurement significantlyand is negligible3.3 Angle Measurement SystemThe angle measurement system provides the missing two piecesof information in addition to the distancethat are innecessary the derivation of 3D coordinates The investigation of the angle measurement systemregarding accuracy and precision have to be carried out by experimental setups that are not influenced bythe other errors of the laser scannerSince the angle measurements cannot be read directly, the requiredinformation regarding angle values have to be derived based on the supportedCartesian coordinatesmeasurement elements ofThe laser beam can be aligned manually towards a target and the anglevalues canbe read However, this alignment is difficult and does not provide sufficient precision Furthermore, a realscanning procedure is more appropriate to assess the angle measurement system since the qualityof theangle measurement system achieved during a scanning process is of interest In the following, a singlemeasurement refers to one scan of a sphere and a mean value is based on several single measurements, lseveral scans of a sphereeGenerally, spheres were used for the investigation procedures and were scanned in different scanningmodes Based on the acquired point cloud, the center points of the spheres can be derived9 Based on theCartesian coordinates of these center points, the spherical coordinates can be easily calculatedhorizontal angles and the vertical angles can be assessed Furthermore,Thus, thethe distance measurement systemhas no influence on the angle measurement system because the horizontal angles and the vertical angles donot change if the distance is varyingAlso other instrumental errors, eg error of collimation axis, error ofhorizontal axis, l e tilting axis, were eliminated by performing measurements in two faces analogous to atheodoliteThe precision of the angle measurement system is defined as the closeness of repeated angle measurementsregarding the local scanner system or, how precisely the anglesof the laser scanner fit to each other Theprecision of the angle measurement system is assessed by repeated measurements to a sphere The resultingspherical coordinates to the center point of this sphere are analyzed by means of the empirical standarddeviation of a single measurement (ss) and the empirical standard deviation of the mean value (sm) Inaddition, the accuracy of the angle measurement system is defined as the deviations of acquired anglemeasurements with respect to nominal angle values derived by a reference system Therefore, the anglemeasurements are described by the angle differences between the horizontal and vertical anglesof twotargets For assessing the accuracy of the angle measurement system, two different experimental setupswere chosenTest Field of Observation PillarsThe first method uses the test field of observation pillars, cf Section 313 The symmetric configuration ofthe pillars takes advantage of covering a full circle from the point of view of the center pillarimental setup was chosen so that the surrounding pillars were occupied by sphereswas set up on the middle one, cf Figure 3 21 The spheresThe exper¬and the laser scanneras well as the laser scanner were levelled Thus,the horizontal positions are accurately guaranteed The vertical height of each sphere is influenced by thepositions of the screws on the tribrach For having the same offset of each sphere in height caused by usingtribrachs, the screws of all tribrachs were set to the sameposition This can be easily done by mountingeach tribrach (supported by a target) on the same pillar and surveying the heightwith a theodolite Anoffset of the tribrach in height can be corrected by moving the screws of the tribrach and shifting the target'Here, the diameters of the spheres were used as well known constants to achieve the best accuracy ('fix' adjustment)