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

92 5. Kinematic Laser Scanninghas a negligibly small eccentricity of the scan center, cf. Section 3.4.1, and since the prism attached to thetop of the laser scanner has a negligibly small offset of Ay, the coordinates of the pillaron which the laserscanner is set up and the coordinates of the prism can be compared. Therefore, the coordinate differences inx- and y-direction define the component Ax of the translation vector between the origin of the laser scannersystem and the prism attached to the laser scanner. The resulting value of the component Ax has an offsetof 0.158 m. The accuracy is sufficient with a value of less than one millimeter.The component Az can be obtained by using the same procedure applied for deriving the componentAx. The only difference is the resulting coordinate difference between the z-values of the prismobservation pillar on which the laser scanner is mounted also contains the height of the originand theof the laserscannersystem. Thus, the height of the instrument has to be considered and subtracted. The resultingvalue of the component Az is an offset of 0.159 m. The accuracy is sufficiently highone millimeter.with a value of less thanTable 5.1 summarizes the translation vector between the prism on top of the laser scanner and the origin ofthe local scannersystem.Table 5.1: 3D translation vector between the prism atop the laser scanner and the origin of the local scanner system.Ax [m] Ay [m] Az [m]0.158 0.000 0.1595.2 Rotation Time of Rotating Mirror of Laser ScannerThe laser scanner isreference is required to synchronizeoperated in the profile mode. For each point measured by the laser scanner, a timethe laser scanner data with data from additional instruments. Themirror of the laser scanner rotates quickly about its horizontal axis and the data acquisitionTherefore, the generation of a time tag for each single laser pointrate is fast.is not recommended. It seems morereasonable to validate if the rotation of the mirror is constant and is, therefore, appropriatefor the use astime base. If there is a constant periodic time for one full rotation about 360 °, the rotation time T can bederived byT=^-. (5.2)The rotation time T then serves as time information.By knowingthis rotation time and the number ofmeasured points for one vertical profile, a time tag can be calculated for each single point measured by thelaser scanner. If a common time base is available, data from different instruments can be synchronized. Forexample, one typical time tag is the PPS signal provided by GPS signals with an accuracy of approximately1 ms. The following sections deal with the determination of the rotation time T. The aim is to specify therotation time with a precision of