# Estimation of the Block Adjustment Error in UAV Photogrammetric Flights in Flat Areas
## 2 Materials and Methods
### GNSS Campaign  
The GCPs and ChPs (check points), also known as ground evaluation points (GEP), were marked before the flight. This was done by creating a cardboard template of a target of 60 cm \(\times\) 60 cm comprising three blades, each separated by 120\({}^{\circ}\), and a central circle. Each point was marked using reflective white paint, and a survey nail was placed in the centre (Figure 3a).  
All the points (GCPs and ChPs) were measured with dual-frequency GNSS receivers (GPS + GLONASS) on a tripod and centred on the point mark (Figure 3b) for at least 15 min. All the points were measured twice with a different constellation and different receivers, which were configured for a static survey. A total of ten GCPs were measured, distributed at the four corners of each block, and nine ChPs were arranged randomly so that there were at least two ChPs in each block. The distribution of the points is shown in Figure 4. When designing this distribution, priority was given to ensuring that the points were located on the firm ground, such as roadways, and that no nearby elements would impede the satellite signal. The GCPs were positioned in the common areas between the blocks. Short cross flights were made in these same areas to optimise the UAV batteries and reduce the number of GCPs.  
The precise geodetic ionospheric correction models of the CODE (Centre for Orbit Determination in Europe [40]) and the precise ephemerides of the IGS (International GNSS Service) [41] were used to calculate the GCPs and ChPs coordinates for both constellations. Data from 22 continuous stations were processed to improve the general network configuration and link the local measurements to a regional geodetic reference framework. These stations are located in the southeast of the Iberian Peninsula and are part of the regional networks of the Region of Murcia (REGAM and MERISTEMUM) and the Spanish National ERGNSS-IGN Network (National Geographic Institute), with 24 h and 30 s of observation over ten days. The GNSS vectors in the network were processed using Leica Infinity software, with absolute antenna calibration models and Vienna Mapping Functions  
Figure 3: GCP: (**a**) target with three blades and a central circle with a nail in the centre; (**b**) dual-frequency GNSS receiver positioned on a tripod and centred on the point mark.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Drone** & **Phantom 3 Pro** & **Inspire 2** \\ \hline Resolution & 4000 \(\times\) 3000 pixels & 5280 \(\times\) 3956 pixels \\ DJI & FC300X & Zemmuse X5S \\ F-stop & f/2.8 & f/1.7 \\ Focal distance & 4 mm & 15 mm \\ Equivalent 35 mm focal length & 20 & 30 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Camera and image specifications for the Phantom 3 Pro and Inspire 2 UAV used in the study.  
(VMF) [42] for the tropospheric modelling. Subsequently, the vectors previously obtained in the network were combined with their complete variance-covariance matrices using Microsearch GeoLab software. This allows the estimation of the whole set of coordinates of the network points on the ETRS89 system, with an independent weighting strategy based on the quality of the vectors following the methodology used in high-precision networks [43; 44].