# Quality Assessment of DSMs Produced from UAV Flights Georeferenced with On-Board RTK Positioning
## 3 Results  
### Empirical Accuracy Assessment of Different Block Control Configurations  
The graphs of Figure 4 depict the mean ground accuracy measured by the RMSE at the 14 checkpoints for each of the eight flights. The accuracy in the horizontal coordinates (RMSE XY) was provided by the RMSE of the horizontal component of the error vector, while RMSE Z refers to the vertical component of the same error vector. On the left-hand side, the bars refer to flights with GNSS positions computed from the local master station (from RTK1 to RTK4); on the right-hand side, the bars refer to positions computed with respect to a CORS Network (from NRTK1 to NRTK4). Table 2 summarises the RMSE ranges in XY and Z on the 14 checkpoints for the different block control configurations.  
Figure 4: _Cont_.  
Figure 3: The check areas listed in Table 1: orange: asphalt; red: roof; green: meadow; blue: ploughed field. Also shown are the location of the GCPs (red triangles) and of the signalised checkpoints (green triangles).  
The **12 GCP** configuration provided an average accuracy in the horizontal and vertical coordinates of 1.5 cm and 2.3 cm, respectively (worst case: 2.0 cm and 3.3 cm, respectively), while the corresponding figures for **RTK** were 2.4 cm and 4.6 cm from a local master RTK mode, and 3.0 cm and 9.5 cm for a NRTK mode. Finally, the **RTK+IGCP** configuration showed horizontal and vertical coordinates accuracy of 2.5 cm and 3.2 cm, respectively (worst case: 3.2 cm and 4.7 cm, respectively).  
Though the results are obviously purely empirical, it is interesting to notice the behaviour of the different configurations when expressing the accuracy range (max RMSE--min RMSE) as a percentage of the worst case (max RMSE). In both the horizontal (XY) and vertical (Z) coordinates, block control with GCPs (**12 GCP**) was the most stable, with 39% and 49%, respectively, in XY and Z; in contrast, when using AAT, only far higher variations are registered: 50% and 85%, respectively, in XY and Z. However, adding one GCP (**RTK+IGCP**) brings the relative variation back to 45% and 61%, respectively, in XY and Z.  
As stated before, the statistics in Figure 4 and Table 2 refer to the 14 checkpoints available. However, the RMSE for the **RTK** and **RTK+IGCP** configurations were also computed using all of the available checkpoints: 26 and 25, respectively. The changes to the RMSE values shown in Figure 4 are in both cases negligible, as they amount to 1-2 mm in most cases, and to 5 mm in one case only.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & \multicolumn{4}{c}{**RMSE XY (m)**} \\ \hline  & 12 GCPm [1] & RTKm [1] & RTKm+IGCP [1] & 12 GCPn [2] & RTKn [2] & RTKn+IGCP [2] \\ Min & 0.012 & 0.021 & 0.021 & 0.015 & 0.021 & 0.017 \\ Max & 0.020 & 0.025 & 0.029 & 0.016 & 0.042 & 0.032 \\ \hline  & & & \multicolumn{4}{c}{**RMSE Z (m)**} \\ \hline  & 12 GCPm [1] & RTKm [1] & RTKm+IGCP [1] & 12 GCPn [2] & RTKn [2] & RTKn+IGCP [2] \\ Min & 0.017 & 0.019 & 0.018 & 0.021 & 0.067 & 0.029 \\ Max & 0.024 & 0.095 & 0.039 & 0.033 & 0.126 & 0.047 \\ \hline \hline \end{tabular} \({}^{1}\) Flights RTK1 to RTK4; \({}^{2}\) Flights NRTK1 to NRTK4  
\end{table}
Table 2: Best and worst RMSE computed on 14 checkpoints for the six sets of flights with the same block control configuration and real-time kinematic (RTK) mode.  
Figure 4: Root mean square errors (RMSEs) on 14 checkpoints respectively for horizontal (**a**) and vertical (**b**) coordinates, for the different flights and ground control configurations. Notice that the horizontal and vertical coordinates error scale ranges differ.