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

30 3. Calibration of Terrestrial Laser Scannerbetween the sphere or the prism, one centimeter deviation in distance does not affect the desired parameterssignificantly, and the theodolite, calibration parameters can be derived by usingangle measurements and simple geometrical relations.the vertical and horizontalEach sphere has different calibration parameters. The closeness of the calibration parameters of all thespheres allowed for the derivation of two sets of mean values for the diameter and the vertical offset (forthe two different sizes). All results showed that the symmetry of the diameter and the central position of theattached adapter are sufficiently adequate. Thus, the spheres, cf. Figure 3.7, can be used for the calibrationof the laser scanner.Figure 3.7: Sphere used as target for laser scanning. The diameter of the shown sphere is approximatelysphere is attached to a prism holder.12 cm. TheThe center point of a sphere can be calculated easily. Normally, a minimum number of four points satisfythe definition of a sphere and with laser scanning, many more points than necessary for defining a sphereare generated. The resulting redundancy allows for the derivation of the center point via an adjustment.Therefore, the diameter of the spheres can be introduced either as an unknown parameteror as a knownconstant. Within the adjustment procedure, blunders are detected automatically. Blunders are presentnormal distances from each point to the sphere surface exceed a user-defined limit, e.g. 5 mm. This parame¬ter is the same as using normalized residuals and is introduced because it can be interpreted geometrically.if theMore information concerning the adjustment procedure can be found in Appendix C.The adjustment isiterated until either all points lie within a maximum distance to the surface of the sphere, e.g. 5 mm, or untilthe unknowns, i.e. coordinates of the center points and the diameter, change below a limit, e.g.The diameters of the two spheres (12 cm and 15 cm) were chosen with respect to practical aspects0.1 mm.and withrespect to the minimum angle increments and to the beam divergence of the laser scanner, respectively. Onone hand, the spheres should be small and lightweight enough to be moved and transported during fieldwork.On the other hand, the spheres have to be large enough to be scanned from distances adapted tothe range interval of the laser scanner.Thus, the aforementioned diameters for the sphereswere chosen.Additionally, the best possible value for the optimal diameter of the spheres can be derived mathematically.[Reshetyuk et al., 2005] found that the optimal diameter for the laser scanners, HDS 3000 of HDS LeicaGeosystems and Imager 5003 of Zoller+Frohlich, has a value of 14 cm.