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

6.3 Kinematic Application: Test Tunnel 121ods and numerical models simulating the rock mass behaviour around the tunnel. Theaim of this studywill be to better assess the capability of laser scanning in monitoring geomechanical processes occurring inrocks.6.3 Kinematic Application: Test Tunnel6.3.1 IntroductionDue to the fast acquisition rate and the high point density, laser scanning isnot onlysuitable for staticapplications but also for kinematic applications. This is especially the case for profile measurements, whichare useful for generating an area-wide surveying of objects by means of a moving platform.The following example of a kinematic application refers to the surveyingof tunnel surfaces. Thegoal isto prove that, with kinematic laser scanning, a sufficient absolute accuracycan be achieved.faces are either scanned with static applications, using consecutive viewpoints,applications with respectto the rail axis. RelativeTunnel sur¬or with relative kinematicsurveying with respect to the rail axis is appropriate fordetecting objects dangerous to trains, i.e. railway loading gauge. However, the interpretationmations is difficult since the object, i.e. the tunnel surface, or the rails could be changedtime.of defor¬or moved withThe kinematic application is performed on a test tunnel, i.e.Zurich. The laser scanner is mounted on the test trolley, which moves alongthe calibration track line at the IGP of the ETHthe track. A total station tracksthe moving platform so that the absolute trajectory can be obtained. The laser scanner surveysvertical 2Dprofiles normal to the moving direction.During the motion, control points installed on both sides of thetrack at different heights are scanned. The 3D coordinates of the control points visualized by spheres canbe calculated. The performance and accuracy are assessed by comparing the calculated center points of thespheres with the reference coordinates based on surveying with a total station. The experimental setup wasalready shown in Section 5.1, cf. Figure 5.1, and the test field of control points alongdiscussed in Section 3.1.2.the test tunnel wasThe test trolley was forced to run with a constant velocity along the test tunnel. The control software for theencoder, causing the rotation of the wheels of the test trolley, allows a maximum velocityof 0.7 . The testtunnel and constant velocities are chosen to minimize errors for calculating the trajectory of the test trolley.Thus, the trajectory can be approximated by a regression line implying a constant velocity. A more generalmethod is the approximation of the trajectory bya Kaiman filter. TheKaiman filter can respond to varyingsystem states such as variation in the velocity. Applying both mathematical tools allow for comparing theresults and assessing the potential of kinematic laser scanning for absolute geo-referencing.The test runs for this kinematic application include• different velocities of the test trolley,• different rotation times of the rotating mirror of the laser scanner, and• different scanning resolutions.The point-spacing along the moving direction can be defined by the velocity of the test trolley and by therotation time of the rotating mirror of the laser scanner. The point-spacing within each vertical profile hasto be controlled by means of the scanning resolution according to the scanning modes supported by themanufacturer of the laser scanner.