## 3 Method Validation
### UAV Photogrammetric Flight  
The photogrammetric flight for the capture of the RGB and TIR images was designed following the established recommendations for image capture with low-cost UAV equipment. The flight began at 9:00 a.m. with the image capture, after a previous wait of 30 min for the stabilization of the sensor. During the flight, the atmospheric conditions were favorable for data collection; the ambient temperature was 21 degrees and the skies were clear with no wind.  
The platform used was a quadcopter multirotor, specifically a DJI Mavic 3T, which has an integrated dual M3T sensor (Figure 3). The main features and technical information of the UAV platform and the dual sensor cameras are shown in Tables 1-3.  
The calibration of the RGB and thermal sensors was guaranteed by the company contracted for the photogrammetric flight. In addition, the self-calibration tool implemented by Agisoft Metashape was applied during the photogrammetric processing of the images  
Figure 2: General overview of the Meruelo MSW landfill and distribution of the different facilities: treatment and recovery plant, landfill, and leachate plant.  
for the generation of the 3D model. This step eliminates possible remaining lens distortions to ensure greater accuracy of the representation.  
The automatic flight for image acquisition was designed using the proprietary software of the UAV to guarantee maximum compatibility. The sensor was configured with an inclination of 30\({}^{\circ}\) with respect to nadir to adequately capture the steep slopes of the landfill. The automatic flight designed and its main specifications are shown in Figure 4 and Table 4.  
Complementarily, in order to guarantee the correct measurement of the surface temperature of the landfill, the thermal sensor was configured with an emissivity of 0.94 [67], which can be considered the average for the entire surface of the landfill due to the homogeneity of the MSW arrangement.  
In addition to capturing the images, a total of 17 GCPs were distributed along the landfill to orient the images, scale, and georeference the model. The GCPs were evenly  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Specifications**} \\ \hline Weight & 920 g \\ Dimensions & 347.5 \(\times\) 283 \(\times\) 107.7 mm \\ Autonomy & 45 min \\ Speed & 15 m/s \\ GNSS & GPS + Galileo + BeiDou + GLONASS \\ \hline \hline \end{tabular}
\end{table}
Table 1: UAV platform specification parameters.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Specifications**} \\ \hline Sensor type & Uncoded Vox microbolometer \\ Resolution & 640 \(\times\) 512 px \\ Spectral range & 8–14 \(\upmu\)m \\ Accuracy & \(\pm\)2 \({}^{\circ}\)C \\ Focal distance & 9.0 mm \\ Image format & JPEG, R- JPEG \\ \hline \hline \end{tabular}
\end{table}
Table 3: M3T thermal camera specifications.  
Figure 3: Image of the DJI Mavic 3T UAV which was used for the flight over the landfill.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Specifications**} \\ \hline Weight & 920 g \\ Dimensions & 347.5 \(\times\) 283 \(\times\) 107.7 mm \\ Autonomy & 45 min \\ Speed & 15 m/s \\ GNSS & GPS + Galileo + BeiDou + GLONASS \\ \hline \hline \end{tabular}
\end{table}
Table 1: UAV platform specification parameters.  
distributed throughout the study area at different heights. The coordinates of the GCPs were taken with GPS using a Leica GS-15 GPS device (Leica Geosystems, San Galo, Switzerland) in an official reference system (ETRS89-UTM Zone 30). Additionally, during the flight, the temperature was measured with a hand-held sensor at 10 geolocated points distributed throughout the study area as ground truth for the calibration and thermal adjustment of the 4D model.