## 3 Results
### Experiment (5): Point-Based Absolute Accuracy and Precision for Multiple Strip Measurements  
The last **Experiments (5)** and **(6)** are investigations based on the second data set already mentioned in Table 3 as multiple strip measurements. Since the analysis of different GNSS master stations in **Experiment (4)** has shown equal results for all master stations, the processing is only done using a VRS. The first parameter for the precision of the point cloud is given with the noise of different parts in the point cloud. The approach for noise estimation is used as in **Experiment (1)** and the areas of the 12 targets are extracted for this estimation. The result for the four repetitions and all targets is a mean noise of 0.6 cm. Here, several factors influencing the accuracy of the point cloud can be evaluated simultaneously. First of all, the influence of the laser scanner on the error budget can again be classified as small, since the manufacturer accuracy is even worse specified than the total noise of the resulting point cloud. Of course, the specified distance for the given accuracy is not yet reached with 25 m flight altitude, but the calculated value is still much lower than the given 1.0 cm. Furthermore, the performance of the optimization by RiPRECISION is good for this data set, since no major offsets remain within the point clouds. The further results in this experiment are derived from the estimated target centers and the comparison with the target centers from the TLS reference point cloud. The difference divided in the east (blue), north (red) and height (yellow) coordinates are shown in Figure 12 in histograms summarising all four flight repetitions. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Mean Absolute**} & \multicolumn{3}{c}{**Root Mean**} \\  & \multicolumn{3}{c}{**Difference [cm]**} & \multicolumn{3}{c}{**Square Error [cm]**} \\ \cline{2-7}  & \(\delta_{x}\) & \(\delta_{y}\) & \(\delta_{z}\) & \(\text{RMSE}_{x}\) & \(\text{RMSE}_{y}\) & \(\text{RMSE}_{z}\) \\ \hline Own master station (pillar) & 1.0 & 1.9 & 3.3 & 1.1 & 1.3 & 1.2 \\ Virtual Reference Station 1 (VRS1) & 0.9 & 1.6 & 2.7 & 1.1 & 1.4 & 1.4 \\ Virtual Reference Station 2 (VRS2) & 0.9 & 1.6 & 2.6 & 1.1 & 1.4 & 1.7 \\ CORS station (SAPOS NRW) & 0.4 & 1.7 & 2.9 & 1.2 & 1.4 & 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Target center difference between TLS reference and UAV point cloud with four different GNSS master stations. The difference is described using the mean value and standard deviation for the East, North and height direction. The determined mean absolute difference result in \(-1.1\) cm for the East, 1.1 cm for the North and 4.3 cm for the height direction with corresponding RMSE of 1.8, 1.0 cm, and 2.2 cm. Therefore, the absolute accuracy described by these differences is consistent with the results from **Experiment (2)** and even a bit smaller for the horizontal direction. The systematic effect in the vertical direction is again similar with a magnitude of 4 cm, which was already recognized in the analysis of the single strip measurement. Since this is only presented as a shift of the point cloud, a simple solution in practice would be the use of one or multiple GCPs. By using a single GCP with precise height information, the entire point cloud can be shifted to the correct height without great effort. The precision described by the standard deviation is given with 0.8 cm for the East, 0.5 cm for North and 2.1 cm for the height component. Both components, horizontal and vertical, are again small and are in accordance with the specification of the manufacturer. Overall, the second data set based on the typical cross-flight pattern with the use of the full processing capability except for the inclusion of ground control information has shown again good results with absolute accuracy described by the mean difference around 1 cm for the horizontal direction and around 4 cm for the vertical direction.