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

3.4 Instrumental Errors 493.4.1 Eccentricity of Scan CenterThe ideal construction of a rotating instrument assumes that the real center of rotation correspondsideal center of rotation. If there is a deviation due to mechanical imperfections,is present. The laser scanner system to be calibrated has two different rotations: a primaryto thethen an instrumental errorrotation aboutan horizontal axis, i.e. tilting axis, and a secondary rotation about a vertical axis, i.e. rotation axis. In thefollowing, an examination of the eccentricity of the laser scanner center is presented. In particular, twodifferent aspectsare discussed:• the eccentricity of the vertical axis• the eccentricity of the scan centerInfluences caused by errors in the horizontal axis are discussed separately,investigations were carried out in such a way that errors caused bycf. Section 3.4.4.All describedthe horizontal axis do not influence theinvestigations. All objects to be scanned, e.g. spheres, were positioned horizontally and at the same height.Thus, errors resulting from the horizontal axis can either be eliminated or they have the same influence onall the object points. Possible errors caused by the distance measurement system, e.g. additive constant,scale factor, also have to be reduced from the measurements.The objective is to verify that the laser scanner can be mounted on a tripod or on an observation pillar, likea total station.This means the laser scanner can be centered on a control point and the rotation center ofthe vertical axis corresponds to the center of this control point. Furthermore, the origin of the local scannersystem has to conform with the rotation center. The setup of a laser scanner can be analogous to that of atotal station. First, a tribrach is mounted on a tripod or on an observation pillar and is then levelled usinga plate level, which is attached securely on the laser scanner. Second, an adapter platetribrach. Third, the laser scanner is installed on this adapter plate and is readyand the rotation axes can be seen in Figure 3.25.is fixed onto thefor use. The laser scannervertical axis— — —horizontal axisFigure 3.25: Principle of the laser scanner and the axes of rotation. The laser scanner has two axes of rotation: aprimary fast rotation about the horizontal axis and a secondaryslow rotation about the vertical axis.Eccentricity of Vertical AxisThe investigation of the eccentricity of the vertical axis covers two aspects: the rotation has to be homoge¬nous, i.e.resulting in a circle, and the center pointof this circle has to fit the nominal center of the controlpoint on which the laser scanner was set up. This examination requires high precision regardingthe sur¬veying of a target that is mounted on top of the laser scanner and represents the rotation of the laser scanner.