# Mobile LiDAR for Scalable Monitoring of Mechanically Stabilized Earth Walls with Smooth Panels
## 5 Experimental Results
### Point Cloud Registration and Quality Control of System Calibration and Direct Geo-Referencing  
Point cloud registration was sequentially executed to align: (1) MLS point clouds captured by the onboard two scanners for each drive run, (2) MLS point clouds from neighboring drive runs, (3) TLS point clouds from three scan stations, and (4) MLS and TLS point clouds. The estimated transformation parameters for the alignment of the point clouds from the two scanners in a given drive run were used to assess the quality of the system calibration procedure (major deviations from zero shifts/rotation angles would be an indication of inaccurate system calibration procedure). To quantitatively assess the quality of the GNSS/INS trajectory, the evaluated transformation parameters when aligning point clouds from different drive runs are used (i.e., significant deviations from zero shifts/rotation angles is an indication of less than optimal GNSS/INS trajectory). In this regard, one should note that the registration of the TLS and MLS point clouds was only performed to ensure that there were uniquely defined local vertical and horizontal directions within the study site (i.e., there is not quality control aspect associated with the derived estimates of the transformation parameters in this case). To evaluate the comparative performance of TLS and MLS-based inspection strategies, the results for three faces (with a total of 76 panels) along facade 2 of the MSE wall (i.e., the facade covering faces 2, 3, and 4) are examined. Two MLS drive runs in opposite directions at the vicinity of the MSE wall were used for this comparative test. Six registration steps were performed. Registration of the Riegl and ZF scans in each drive run was conducted in the first and second steps of the analysis. In the third step, alignment between the combined/registered scans from the two drive runs was achieved. Then, three TLS scans were registered to a unified coordinate system in two successive steps. Finally, the TLS and MLS point clouds were registered to the MLS reference frame. For qualitative evaluation, Figures 15-18 illustrate four vertical profiles--which were manually extracted to highlight the alignment quality of point clouds from the two scanners (i.e., Riegl and ZF) in a given drive run, point clouds from two drive runs in opposite directions, point clouds from the TLS scan stations, and point clouds from the TLS and MLS units. The profiles in these figures verify the good alignment among the different point clouds following the proposed registration process. Figure 15: Vertical profiles illustrating post-registration alignment quality of Riegl and ZF scans Figures 1–4 along facades 1 and 2 of the MSE wall. Figure 16: Vertical profiles illustrating post-registration alignment quality of point clouds from two MLS drive runs covering faces 1, 2, 3 and 4 along facades 1 and 2 of the MSE wall. Figure 17: Vertical profiles illustrating post-registration alignment quality of two TLS scans covering faces 2, 3 and 4 along facade 2 of the MSE wall. Figure 18: Vertical profiles illustrating post-registration alignment quality of TLS and MLS point clouds covering faces 2, 3 and 4 along facade 2 of the MSE wall. Table 1 reports the corresponding transformation parameters, square root of a posteriori variance factor (\(\hat{\sigma}\)'), and average RMSE of the normal distances between the registered point clouds in Figures 15-18. The reported values in this table can be used to quantitatively evaluate various aspects related to the used MLS as well as the proposed processing framework. Close inspection of Figures 15-18 and Table 1 shows the following:  
1. The magnitudes of the estimated transformation parameters for the alignment of the point clouds captured by the Riegl and ZF scanners in a given drive run--as indicated by the small values in Table 1a where the estimated parameters are in the range of 2 cm and 0.1\({}^{\circ}\)--confirm the high quality of the system calibration procedure. 2.