# Estimation of the Block Adjustment Error in UAV Photogrammetric Flights in Flat Areas
## 2 Materials and Methods  
The study was conducted in Lorca (Murcia, Spain). The study zone has been continually monitored for ten years. It presents important subduction of the terrain due to the intensive overexploitation of the local aquifers, which has led to deformations in the vertical and horizontal components of up to 10 cm/year [35; 36; 37; 38]. Prior to the flights, NOTAM information for the area to be flown was reviewed. The information was consulted on the geoportal dedicated to drone flights of the Spanish Aviation Safety and Security Agency ([[https://drones.enaire.es/](https://drones.enaire.es/)]([https://drones.enaire.es/](https://drones.enaire.es/)) accessed on 15 May 2021). A UAV photogrammetric survey was carried out during the field campaign (May 2021), dividing the area of interest into four blocks to cover a total area of 80 ha (Figure 1); the areas of each block are shown in Table 2, with the type of UAV used.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **GCP** & **AREA (ha)** & **Ratio GCP/ha** & **RMSE (cm)** \\ \hline
[2] & 3 & 0.02 & 150 & 52 \\
[29] & 45 & 1 & 45 & 0.23 \\
[27] & 5 & 0.83 & 6 & 6.2 \\
[25] & 27 & 5.0 & 5.4 & 6.6 \\
[23] & 7 & 1.5 & 4.6 & 51 \\
[30] & 5 & 2.73 & 1.83 & 3 \\
[31] & 15 & 12 & 1.25 & 14.3 \\
[32] & 20 & 17.6 & 1.13 & 3.6 \\
[24] & 15 & 17.6 & 0.85 & 5.8 \\
[26] & 11 & 37.4 & 0.29 & 5.9 \\
[33] & 6 & 38 & 0.15 & 1.3 \\
[28] & 102 & 1200 & 0.1 & 1 \\
[34] & 9 & 270 & 0.03 & 3.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Studies on the number of GCP and their relation with the study area.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Block** & **Strips Type** & **Area (ha)** & **UAV** & **Flight Height** \\ \hline Block 1 & Forward strips & 19.11 & Phantom 3 Pro & h\({}_{\text{f}}\) = 120 m \\ Block 1 & Cross strip 1 & 0.8 & Inspire 2 & h\({}_{\text{c}}\) = 110 m \\ Block 1 and 2 & Cross strip 2 & 0.8 & Inspire 2 & h\({}_{\text{c}}\) = 110 m \\ Block 2 & Forward strips & 19.11 & Phantom 3 Pro & h\({}_{\text{f}}\) = 120 m \\ Block 2 and 3 & Cross strip 3 & 0.8 & Inspire 2 & h\({}_{\text{c}}\) = 110 m \\ Block 3 & Forward strips & 22.95 & Inspire 2 & h\({}_{\text{f}}\) = 120 m \\ Block 3 and 4 & Cross strip 4 & 0.8 & Inspire 2 & h\({}_{\text{c}}\) = 110 m \\ Block 4 & Forward strips & 18.86 & Phantom 3 Pro & h\({}_{\text{f}}\) = 120 m \\ Block 4 & Cross strip 5 & 0.8 & Inspire 2 & h\({}_{\text{c}}\) = 110 m \\ \hline Total & & & 84.03 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Area in hectares for each block studied; h\({}_{\text{f}}\) refers to the forward flight height, and h\({}_{\text{c}}\) refers to cross flight height.  
During the flights, there was a NOTAM in force in the area affecting Flight Level 100/Flight Level 220, issued by the General Air Academy based in San Javier (Murcia), so we had to coordinate operations with the control tower.  
Two multirotor UAVs were used to acquire photogrammetric data: a DJI Phantom 3 Pro (Figure 2a) and a DJI Inspire 2, equipped with a Zenmuse X5S camera (Figure 2b). The camera specifications are shown in Table 3. The flights were made under a VLOS (Visual Line of Sight) operational scenario with the visual scope of the UAV, using the Dronedeploy application for the flight plan [39]. The meteorological conditions for the flights were optimal: a sunny day with calm winds.  
Figure 1: Location of the study area in Lorca, Via Camino de Puente Alto; the coordinates for the reference system are UTM 30N ETRS89. Points 7,8,9,10,33 have the maximum deformation in the study of [35].