## 4 Discussion
### Flight Scenarios  
Throughout the four analysed scenarios, the RMSE values of the CPs are higher than the GCPs, as these points represent the positioning accuracy of the GCPs. Thus, the lowest residuals are always achieved in the GCPs [44; 66]. On the other hand, [PERSON] et al. [39] reported GSD 0.75 cm/pix for an area of 1.7 ha, using nadir images and a ratio of 3 GCPs/ha. Our study found GSD 2.7 cm/pix, using 0.12 GCP/ha for an area of 0.8 km\({}^{2}\). This difference is mainly due to the variation between the flight heights of each study since [PERSON] et al. reported a flight height of 45m, and we used a flight height of 120 m. However, when comparing the vertical and horizontal RMSE results for the CPs, Scenario A of this study presents 4x and 1.8x the GSD, resulting in better than the results reported in [39] using a mechanical shutter.  
Scenario B showed the best results compared to the other scenarios in this research for the CPs in the vertical RMSE. Scenarios B and D have oblique perimeter images, which,  
Figure 7: Scenario B RMSE in the CPs for the six configurations analysed.  
in different studies, have shown an improvement in vertical accuracy [38]. However, Scenario B presented better results than Scenario D, considering that the reduction of systematic errors is attributed to increased angles between homologous rays [67]. Scenario B, composed of forward strips combined with oblique perimeter images at the block edges, produced the lowest vertical RMSE for the CPs, 2.5x the GSD, and a horizontal RMSE equal to 1.7x the GSD. Scenario D presented the vertical RMSE for the CPs at 4.0x the GSD and a horizontal RMSE equal to 2.0x the GSD. A difference between Scenarios B and D was that Scenario D combined various strips with a significant overlap in the same project, which could negatively influence the result. Similarly, Scenario D had a higher battery consumption and required more time for post-processing than Scenario B.  
According to reported studies in flat areas where the combination of nadir and oblique images was used (Table 1), Scenario B presents similar results in the value of the RMSE concerning [36; 40]. These studies reported a lower flight height than ours and a higher number of GCP/ha. Therefore, using Scenario B, resources can be optimised, and larger areas can be covered. Furthermore, we confirmed that oblique images provide a major key point coincidence per image for studies in flat areas, improving vertical accuracy [37; 64; 68]. However, [PERSON] et al. (2022) [37] presents the best results overall. The main difference is using oblique images through a special five-camera UAV with a microcontroller device. This implies using non-conventional UAV equipment specially designed for this type of photogrammetric survey.  
Another parameter evaluated was the accuracy of the horizontal RMSE. Scenario C, which comprises forward strips and a cross strip at the edges of each block where the GCPs have been located, presented the lowest RMSE in the horizontal direction for the GCPs and CPs (Table 6). These results are consistent with [57; 58; 59], where the authors hypothesise that the cross strips improve accuracy in the photogrammetric restitution, using less GCP. On the other hand, Scenario A, although it is the most used in UAV-DAP, presents more unfavourable results than those obtained in Scenarios B and C. The same situation occurs in Scenario D.