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

3.1 Laboratories and Tools for Calibration 29and can be received by the frequencymeasurement unit. Basedon this received standardized frequency,an absolute time base for the frequencycounter can be defined.The modulation frequency of a sensor is transmitted to an APD. The detected signal is amplifiedand de¬modulated. The frequency counter calculates the detected modulation frequency. Additionally,ulated signal can also be visualized by an oscilloscope. The electronic unit for frequencyshown in Figure 3.6. The APD, the frequency counter, the amplifier and the oscilloscopethis figure.the demod¬measurement iscan also be seen inFigure 3.6: Electronic unit forfrequency measurement including an APD, an amplifier, a frequency counter, and anoscilloscope (from right to left).3.1.5 Calibration of SpheresIn contrast to traditional geodetic instruments, e.g.total station, GPS, levels, a laser scanner does notsupport the direct determination of coordinates of discrete points. For the calibration purposes of a laserscanner, itis essential to work with discrete points to derive the coordinates of these points. Several geo¬metrical objects are suitable for laser scanning, e.g. planar coded targets, cylinders and spheres.appropriate object for laser scanning in terms of deriving coordinates of a discrete point is a sphere.advantages of a sphereare manifold. First,each sphere is absolutely defined in 3D bythe center point and its diameter. Second, a sphere is featured by a homogenousThe mostThethe coordinates ofsurface and is invariantregarding the viewing angle. Thus, spheres are well-suited for use as targets for laser scanning. However,there is one major disadvantage. The angle of incidence worsens with increasingand vice versa. The introduction of weights can reduce this problem. However, these pointsas they define the geometry of the sphere.distance from the centerConsequently, massive wooden spheres in two sizes were prepared as targets for laser scanning.are essentialThe diam¬eters are of approximately twelve and fifteen centimeters. Adapters were fixed on the spheres for attachingthem to prism holders. When installing the adapters, caution was taken to avoid an offset in position andin height between the prisms and the spheres. Furthermore, the spheres were paintedwith a white colourfor achieving high intensity values for the reflected laser beams. Overall, each sphere had to be calibratedwith respect to• the diameter and the symmetry of the diameter, in horizontal and vertical direction,• the central position of the adapter, and• the vertical offset between prism center and sphere center.The calibration processwas performed by usinga theodolite (Wild T 2000). The theodolite and the spherewere set up on two pillars of the test field of observation pillars, cf. Section 3.1.3. By knowingthe distance