# Testing Accuracy and Repeatability of UAV Blocks Oriented with GNSS-Supported Aerial Triangulation
## 4 Conclusions  
An empirical study on the accuracy and repeatability of UAV block orientation by GNSS-AT, carried out using three different software packages on four repeated survey flights, has been performed. An average horizontal RMSE of 2.2 cm has been obtained by GNSS-AT with all packages and blocks, compared to 1.5 cm with the traditional orientation with GCP; in elevation, the values are 5.5 cm and 2.1 cm, respectively: a dense ground control, therefore, still provides better accuracy. However, the results found are in line with the claims of the manufacturer and suggest that, as far as control of horizontal displacements is concerned, periodic photogrammetric surveys can rely on on-board RTK or PPK GNSS positioning to define a stable reference system to detect changes of well defined points exceeding 5 cm. As already found by other authors, in elevation, though, biases of several cm may arise, which can be controlled using at least one GCP. The location of the additional point and, when the conditions on site allow for it, the additional benefit of using more (e.g., three) well distributed points might be worth investigating. In our four test flights (see Table A5 in Appendix A), fixing three GCP along the block border slightly improved the results compared to just one in the middle. It has been found that the influence of the standard deviations assigned to the camera projection centers on the horizontal accuracy is generally negligible, independently of the software package. In elevation, however, the picture is less clear, with different trends and differences in accuracy that can be relevant, if not in absolute terms at least in percentage. Currently, we do not have solutions to suggest for this problem, apart from maintaining that different standard deviations for XY and Z should be used, since this is inherent to satellite positioning. As far as the \"best\" standard deviations to assign are concerned, no indications can be provided, as this depends on the specific hardware and software, system calibration and satellite configuration. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**MEAN DZ (m)**} \\ \hline  & \multicolumn{2}{c}{**RTK1**} & \multicolumn{6}{c}{**RTK2**} \\ \hline  & **12 GCP** & **RTK** & **RTK+IGCP** & **12 GCP** & **RTK** & **RTK+IGCP** \\ \hline
**Pix4D** & 0.024 & 0.000 & 0.046 & **Pix4D** & \(-\)0.041 & 0.000 & 0.083 \\
**PhotoScan** & 0.025 & 0.000 & 0.000 & **PhotoScan** & \(-\)0.017 & 0.000 & 0.007 \\
**MicMac** & \(-\)0.137 & 0.15 & \(-\)0.006 & **MicMac** & 0.010 & 0.256 & 0.154 \\ \hline  & \multicolumn{2}{c}{**RTK3**} & \multicolumn{6}{c}{**RTK4**} \\ \hline  & **12 GCP** & **RTK** & **RTK+IGCP** & **12 GCP** & **RTK** & **RTK+IGCP** \\ \hline
**Pix4D** & \(-\)0.025 & 0.000 & 0.079 & **Pix4D** & \(-\)0.043 & 0.000 & 0.025 \\
**PhotoScan** & \(-\)0.083 & 0.000 & 0.003 & **PhotoScan** & \(-\)0.033 & 0.000 & \(-\)0.001 \\
**MicMac** & \(-\)0.028 & 0.06 & 0.167 & **MicMac** & \(-\)0.146 & 0.122 & 0.034 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Means of the residuals of the projection centers elevations for the three configurations in each flight. Camera calibration in UAV blocks oriented with GNSS-AT was not meant to be the main topic of the paper. However, on-the-job calibration is likely in our opinion to be the best operational compromise, with the awareness that unless at least one GCP is provided, there might be biases in object coordinates, especially in elevation. Three software packages have been used, not to find which one is the best, but to highlight that there may be differences in GNSS-AT implementation and therefore different outcomes. The analysis of the influence of the standard deviations and the residuals on the camera stations show that indeed the treatment of the observation weights among the three packages is different. As far as the horizontal coordinates are concerned, this does not seem to lead to remarkable differences; in elevation, though, this might not always be the case.