# Estimation of the Block Adjustment Error in UAV Photogrammetric Flights in Flat Areas
## 4 Discussion  
The planning of the UAV-DAP flights is key for obtaining the centimetric level of accuracy. One aspect to consider is the accuracy of the GCPs, being recommended to be less than the GSD. The mean in this study was 7 mm, which was less than the GSD. The GCPs and ChPs must also be located in places visible in the photo acquisition and must not impede the GNSS signal. The study area was divided into four blocks, and four different scenarios were analysed. Scenarios A and B generated error results above the GSD. Scenario C, which has cross strips at the edges of the block where the GCP are located, produced the highest accuracy in planimetry and altimetry. These results reveal that option C, with a GCP at each corner and a cross strip at each end of the block at a lower height (less than 10 m), delivered an RMSE below the GSD in the four blocks studied, confirming that the cross strips at the edges of the block increase the accuracy, in aerial triangulation flights with GNSS and INS (Inertial Navigation Systems) [31], up to 23%, if compared the ChPs using conventional flight planning software (Scenario A vs. Scenario C, Table 7). Scenario D, which contemplates the sum of all the previous scenarios, did not show any significant improvement, all the way around, due to the excessively high overlap among the three crossing strips. As it has a greater number of photographs with surplus cover in the cross strips, it requires longer for both the data acquisition and processing phases. It can be confirmed that the RMSE of the ChP is more than two times higher than the GSD and is smallest in Scenario C, where it has a better performance in three of the four cases. The four scenarios have acceptable accuracies in aerial photogrammetry, as they are within the range of 1-2 GSD in planimetry and 2-3 GSD in altimetry [28]. This is thanks to the quality of the data collection on the ground, as well as the planning and processing strategies. A comparison of the results in Table 1 by the authors of [26], with a ratio of 0.29 GCP/ha, similar to this study with 0.2 GCP/ha, shows a decrease in the value of the RMSE, which falls from 5.9 cm to 2.9 cm for Scenario C; this result can be explained by the improvement obtained when the cross strips are made perpendicular to the block. The study in [44] also shows that for the best results in planimetry, the GCP must be located at the corners of the study area, and to improve the accuracy in the vertical component, it is necessary to use a well-distributed GCPs density between 0.5-1 GCP/ha; however, the GCPs must be located at the edges of the block, as the points measured in the centre do not produce any significant improvement in the adjustment, as evidenced in the study. The research of [43; 44] conclude that to improve the vertical and horizontal accuracy, it is advisable to use the GCPs at the ends of the block, as can also be deduced from this research. However, other studies with UAVs [14; 16] demonstrate that a higher number of images affects the accuracy of the photogrammetric survey. The objectives, such as the type of information required, must be established before planning the flight to obtain results that comply with the tolerance of the work and avoid unnecessarily exceeding the number of photographs taken with the UAVs, the flight height, or the number of GCPs. Another aspect worth considering is that in the literature on photogrammetry, there is a clear consensus as to the quantity and location of the GCP, where the accuracy can only be compensated with chains of GCPs perpendicular to the flight strips, or by employing cross strips at the ends, while leaving the GCPs at the corners or on the perimeter of the block. This produces better results for RMSE in regards to east, north, and altitude, as shown in Scenario C.  
In this investigation, the same calculations were recomputed, adding a 5 th GCP in the centre of each block, to check whether or not the incorporation of an additional GCP might have yielded a significant improvement.