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

118 6. Applications of Terrestrial Laser Scanning6.2.3 ResultsRock Mass Structure CharacterizationThe point cloud resulting from scanning the area of the tunnel, which is found to be intersected by severalgeological discontinuities, is shown in Figure 6.10. The figure is a close-up of the point cloud illustratingone of these discontinuities in the upper part of the tunnel face. In this example, the average point spacingof the point cloud is about 5 mm. Comparing the image generated by the laser scanner with a digitalimage representing the same area, cf. lower part of Figure 6.10, it can be seen that in this case, the densityof the point cloud is sufficient to produce a realistic rendering of the surface, thereby allowingidentification of geological features of interest.for theFigure 6.10: Geological discontinuity intersecting the tunnel face, selected points on the discontinuity surface, mea¬surement principle of the discontinuity trace length (up) and corresponding digital image (below).Furthermore, points that were selected to define and locate the discontinuity planeFigure 6.10. The orientation of the discontinuity was then determined by calculatingcan also be seen inthe orientation ofthe best-fit plane minimizing the mean square distance to all the selected points. Dip values of 50 ° and46 ° and dip direction values of 146 ° and 156 ° were obtained through the analysis of the pointcloud andon-site manual measurement, respectively. The measurements based on the laser point cloud are actuallymore representative of the overall orientation of the structure since manual measurements using a compassare directly influenced by local variations of the surface morphology. The geometry of these variations,which correspond to the large-scale roughness of the discontinuity surface, can be quantified easily usingthe distance between the best-fit plane and the selected points. Finally, the trace length, e.g. the length ofthe linear feature resulting from the intersection between the rock face and the discontinuity, is quantifiedby measuring the distance between two points selected on both extremities of the trace. By repeating thisprocess with other discontinuities visible in the image it is possible to producea database that can be furtherutilized to characterize and model the structure and the behaviour of the rock mass around the excavation.Displacement MonitoringThe expected accuracy of displacementpreviously.data both for total station and for laser scanner has been discussedTotal station surveying was used to provide a high order of accuracy,within 1 mm for bothcoordinate differences and 3D point differences, while the expected accuracy of laser scanning was 5 mm forthe coordinate differences and 9 mm for the 3D point differences. For example,the coordinate differences inz-direction of some points (points 1 and 6 of the arrays 100, 200,300 and 400) located at the top of the nicheare presented in Figure 6.11 and in Figure 6.12.and the lower parts show the results obtained by laser scanning.The upper parts show the results obtained by total stationIn all the figures, an upwardtrend ofthe coordinate differences in z-direction between sessions 10 and 11 are visible. The coordinate differencesare defined by subtracting session i from session 0. Thus, the upwardtrend of the coordinate differences