# Quality Assessment of DSMs Produced from UAV Flights Georeferenced with On-Board RTK Positioning
## 3 Results
### DSM Repeatability and Accuracy  
As mentioned in Section 2.7, the 24 DSM were grouped into six homogeneous sets for the repeatability analysis. Figure 5 shows the means and standard deviations in meters of the cell-wise DSM elevation ranges for each set, both overall and according to each surface class (see Table 1), which were computed in the check areas shown in Figure 3.  
The mean value of the discrepancy ranges, irrespective of the surface type, was almost the same, about 6 cm, for the **12 GCP** and **RTK+1 GCP** block configurations. In the **RTK** configuration, the mean was 12.5 cm and 9.5 cm for the local master and the Network RTK modes, respectively. Differences between surface type means were almost negligible, a few mm at most; the average range  
Figure 5: Mean values (**a**) and standard deviations (**b**) of the cell wise DSM elevation ranges for each surface class by each block control configuration and RTK mode (**m** = master, **n** = network).  
as a function of the block control configuration fairly reproduced the trend seen for the RMSE in Z for the checkpoints, though obviously with larger values.  
The ranges standard deviations varied between 2-3 cm, irrespective of the block control configuration and the surface type, with the partial exception of the ploughed fields class, where values were larger than in the other classes and, surprisingly, the **12 GCP** cases performed worst.  
The ranges spatial distribution for each configuration over the whole survey area are shown in Appendix B (see Figures A1-A6) as colour-coded maps of the pixel-wise ranges of elevations of DSM obtained from four blocks that were adjusted under the same control configurations and the same RTK mode. Areas with ranges exceeding 1 m, which are blanked in the figures, correspond to trees and bushes, (some) building roofs sections where matching failed, and car park plots that were empty or occupied in at least one of the flights.  
Figure 6 shows two pairs of graphs, each with four terrain profiles drawn on the DSM. The former graph pair refers to a meadow close to the western border of the block, which originated from the four flights with the local RTK mode. The latter refers to a car park located in the middle of the test field, and originated from the four flights with the local NRTK mode. In each pair, the first graph depicts the profiles on the DSM derived from the **RTK** configuration (a1, b1), while the second one refers to the DSM derived from the **RTK+1 GCP** configuration (a2, b2).  
Figure 7 depicts the mean values (a) and standard deviations (b) of the DSM error for each block control configuration, which were computed on about 1600 GNSS points measured on roads, car parks, and meadows.  
Figure 6: Two pairs of terrain profiles from the DSM generated with the **RTK (a1, b1)** and **RTK+1 GCP (b1, b2)** configurations, respectively. The graphs of each pair refer to the same profile, and show the effect of enforcing the GCP in the **RTK+1 GCP** configuration.  
Figure 7: _Cont._  
The mean error varied in a range smaller than 4 cm for all of the **12 GCP** cases, and in a range of about 9 cm with the **RTK+1 GCP** configurations. With the **RTK** configuration, the mean error varied in a far larger range: from +1 cm to \(-\)14 cm; all of the large errors were negative (i.e. DSM elevations were lower than they should have been). The standard deviations were fairly homogeneous, and varied between 4.0-5.7 cm.