## 2 Materials and Methods
### GNSS Campaign  
On May 16 and 17, 2022, a GNSS data collection campaign was carried out, consecutively to the UAV campaign. The primary aim of this campaign was to obtain the coordinates of the GCPs and CPs using the GNSS geodetic positioning technique. The georeferencing of these points follows the methodology described by [45; 49; 50]. The GCPs were located on the edges of the block, and the CPs were distributed in the central part as in [PERSON] et al's study. [45]. Prior flights, GCPs, and CPs were marked with a 60 \(\times\) 60 cm template using reflective white paint (Figure 2C,E). The centre was marked with a survey nail, which will be reused in future campaigns. The observations were made using Topcon Hiper dual frequency GNSS receivers (GPS (Global Positioning System) + GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema), having a nominal precision of 3 mm + 0.1 ppm horizontally and 3.5 mm + 0.4 ppm vertically in post-processing static (PPS) GNSS relative mode. The antenna was mounted on a tripod, and centred and levelled on the mark on the point (Figure 2D). A total of ten GCPs and nine CPs were measured. The observation time at each point was 15-20 min per session, with a double session observation at each exchanging receiver between both sessions in order to avoid any bias associated with antennas or receivers. The observation method used was the relative static method by carrier phase differences. The choice of this GNSS observation method, although much more laborious both in the field and in the laboratory than RTK methods, allowed us to obtain a level of precision one order greater than that of the points to be compared. Special care was taken to ensure that the GCPs were on the edges of each block and that the CPs were distributed in the central part. The meteorological conditions during the GNSS campaign were as follows: the average temperature was 22 \({}^{\circ}\)C, with no precipitation on either day. The average wind speed was 1.5 km/h, and the average relative humidity was 48%. Figure 2: (**A**) Flat area study zone; (**B**) Inspire 2 UAV; (**C**) 60 \(\times\) 60 cm template with reflective paint; (**D**) GNSS with tripod focused on the centre of the template; (**E**) the mark at a height of 120 m; and (**F**) an oblique photo taken on the perimeter. GNSS Processing  
To ensure greater precision in the results, precise geodetic models of antenna calibrations and ionosphere correction were downloaded from the CODE (Centre for Orbit Determination in Europe). In addition, precise ephemerides from the IGS (International GNSS Service) for the GPS and GLONASS constellations were used, as these two constellations were the ones used by the receivers in the campaign. With these GNSS data recorded in the field, data were also processed adding 22 continuous stations in the southeast of the Iberian Peninsula belonging to the regional networks in the Murcia Region (REGAM and ME-RISTEMUM) and ERGNSS-IGN (National Geographic Institute), with 24-h of observation forming 30 vectors during fifteen days. This was performed to tie the local measurements to a stable regional reference framework. This group of reference stations includes the continuous stations ALHA, LRCA, and MAZA. These three CORS (Continuous Operating Reference Station) are between 3.8 km and 31.3 km from our area. The choice of three permanent stations allows us to have greater repeatability of the solutions in case one of them malfunctions in future campaigns. Of course, in the adjustment of the network, the different weighting of the baselines based on the distances and the variances in the determinations of the baselines were taken into account. The final adjustment of the network was loosely constrained, taking into account the regional velocity field developed previously in the study by [PERSON] et al. [49]. The GNSS vectors were processed with the Leica Infinity (V3) software [51], using absolute antenna calibration models and VMF (Vienna Mapping Functions) [52].