# A Method for Efficient Quality Control and Enhancement of Mobile Laser Scanning Data
## 5 Results
### Ground Truth Verification with TLS Point Clouds and GCPs  
In contrast to the highway, the rural road next to the retaining wall is generally less busy. RW2, therefore, proved to be an ideal test field for us where we could perform reference measurements with a TLS and a total station. We used a Leica RTC360 to acquire laser scans from eleven setup points, which were then registered and referenced by GCPs in the UTM33N reference system. Although we cannot claim that the TLS point cloud is an order of magnitude more accurate than MLS point clouds, we still adopt it as our ground truth. Figure 16 shows three point cloud comparisons: first, the comparison of RTC and MLS point cloud (**left**); second, after a best-fit has been calculated (**middle**); and third, after our correction approach has been applied (**right**). According to the difference pattern, the entire MLS point cloud is deformed inhomogeneously relative to the RTC360 laser scans. For this reason, a best-fit rigid-body registration, as calculated in a commercial software, fails to align the point clouds. Our approach largely succeeds in matching the two point clouds. However, minor deviations remain in the upper part of the retaining wall due to the systematics caused by the small angle of incidence.  
The structure in RW2 is subject to a semi-annual deformation monitoring campaign. There exist numerous retro-reflective targets on the structure and reference targets in the surrounding area. We take advantage of the high coordinate quality resulting from a network adjustment by replacing prisms with dedicated scan targets that preserve the exact GCP position. We used ten circular targets with a diameter of 11.4 cm (4.5 inch) for those points on the retaining wall (red points in Figure 17) and three rectangular targets (30 cm \(\times\) 20 cm) for the ground points (green points in Figure 17).  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **I** & **II** & **III** & **IV** \\ \hline I & - & 22 \(\pm\) 17 & 15 \(\pm\) 13 & 21 \(\pm\) 18 \\ II & 3 \(\pm\) 3 & & 8 \(\pm\) 8 & 18 \(\pm\) 17 \\ III & 3 \(\pm\) 3 & 7 \(\pm\) 7 & - & 21 \(\pm\) 16 \\ IV & 4 \(\pm\) 4 & 3 \(\pm\) 4 & 3 \(\pm\) 3 & - \\ \hline \end{tabular}
\end{table}
Table 4: Statistics of MLS point cloud distances for all combinations before (not colored) and after correcting the urban scene (green): median \(\pm\) scaled median absolute deviation (MAD) in millimeters.  
Figure 16: Point cloud comparison between RTC and 802 point cloud (**left**), after performing a best-fit registration to 802 (**middle**) and after applying our profile-wise correction (**right**).  
It turns out that it is difficult to accurately select or to estimate the center in the MLS point clouds due to their coarse sampling of the laser scan targets. Thus, in addition to the uncertainty of georeferencing, the point selection process inevitably introduces additional systematic errors. We calculate the discrepancies between the reference coordinates of the GCPs known from geodetic measurements and the selected coordinates in the MLS point clouds. Since the MLS point clouds provide timing information for the selected points, we contrast the results of both the GCP and our method in Figure 18.  
The transformation parameters have less noise than those of RW1. The estimated standard deviations of RW2 are also an order of magnitude smaller. We transformed 802 with the plotted transformation parameters to obtain the corrected point cloud in Figure 16 right. The uncertainty of the point selection process becomes evident when looking at the GCP deviations in the left column of Figure 18. As a result, we can rely on the component pointing in the normal direction of the targets only. Therefore, it is not surprising that, due to the orientation of the retaining wall (cf. Figure 14 right), the \(y\) component of both methods matches best. The z-component of the wall targets, on the other hand, does not fit as well, particularly in the middle of the wall.  
Figure 17: Location and type of GCPs at RW2.  
Figure 18: _Cont._