# Testing Accuracy and Repeatability of UAV Blocks Oriented with GNSS-Supported Aerial Triangulation
## Results and Discussion
### eBee RTK Survey Accuracy Assessment  
The mean errors and the standard deviation of the errors, computed at the 14 CP for all blocks, configurations and software packages are reported in Appendix A in Tables A3 and A4. Figure 3 depicts the RMSE of each coordinate for all the combinations while Table 3 contains the mean error for the Z coordinate. In all flights and with all packages the RMSE obtained in the traditional block orientation by GCPs is in the range from 1.1 to 2.0 cm for the combined horizontal coordinates (equivalent to 0.5-0.8 GSD), while the elevations are about 1.5 times less accurate (from 0.7 to 1.3 GSD). There is little or no bias, since the mean error is always better than 1 cm in XY and never larger than 1.1 cm in Z (see Table A2). Looking at the results of **RTK**, it is apparent that the accuracy of GNSS-assisted AT with the eBee RTK is on the one hand remarkable and often on the par with that obtained with ground control, especially in horizontal coordinates, where the combined RMSE in XY ranges from 0.6 to 1.2 GSD. On the other hand, comparing the **12 GCP** results with those of **RTK**, it is evident that there might be problems with heights: here the RMSE in Z ranges from 0.8 GSD (almost as good as the best  
Figure 3: RMSE at 14 checkpoints for the three configurations: BBA with GCP only (**12 GCP**), with GNSS-AT only (**RTK**) and with GNSS-AT with the addition of 1 GCP (**RTK+1 GCP**). Results are presented for each flight and for each software package. **12 GCP**) to 4.0 GSD. In two cases (RTK1 and RTK4) both the XY and the Z accuracies of **RTK** are very satisfying, with average residuals no more than 5 mm larger than those of **12 GCP**. In other words, the RTK solution practically achieves the same accuracy found using GCP as far as the horizontal coordinates are concerned. In elevation, to the contrary, there are three cases with very good agreement (with RMSE better than 2.5 cm) and cases where the accuracy is worse, up to 9.5 cm in PhotoScan, 6.8 cm in MicMac and 8.5 cm in Pix4D, see Table 2. Here, a bias is apparent, since the mean error is markedly different from zero, with either positive or negative values for the same flight, depending on the software package. Figure 16 in Appendix A shows the plots of the errors obtained with PhotoScan on all 26 available checkpoints for the RTK configuration and for all four flights. In the horizontal coordinates, the error spatial distribution does not show any global systematic pattern, though in three of the test flights, in the eastern side of the block, a residual component parallel to strip direction is apparent on most points. As far as the elevation is concerned, it is hard to find any similarity between the four flight patterns as well as any pattern at all within a particular flight. As the error in elevation turned out to be higher than expected (in particular in RTK2 and RTK3), in **RTK+1 GCP** an extended BBA has been computed, using as control information the projection center coordinates and those of GCP No. 13 (see Figure 1) located in the middle of the block. As shown in Table 3 and in the diagrams of Figure 3, the elevation accuracy improves considerably, down in the worst case to slightly more than 3 cm. Where the statistics in elevation were good already, such as for the Pix4D and PS solutions in RTK1 and RTK4, adding the GCP brings small changes (less than 1 cm) to all coordinates (some for the better, some for the worse). With the additional GCP, in the RTK1 block with MicMac a previous elevation RMSE of 8.5 cm is down to 1.4 cm. In other words, with at least a GCP the bias is removed or greatly reduced in all cases: the RMSE in Z now ranges from 0.6 to 1.6 GSD. As far as horizontal coordinates are concerned, the changes are very small, in the order of a few mm, except for RTK2 with PhotoScan and Pix4D, where they reach 1.2 cm; the combined RMSE in XY now ranges from 0.7 to 1.2 GSD.