## 2 Methods
### Data and Thermal Imaging Capture with Low-Cost UAVs  
Images are the fundamental basis of photogrammetry for the generation of three-dimensional models or orthoimages [41]. In the field of UAVs, images are generally captured on the basis of preprogrammed automatic flights that are designed according to the needs of the project objective [42]. At present, despite the losses in sensitivity and accuracy associated with low-cost UAVs, they are the most widely used alternative in several investigations [43]. This study proposes the use of dual sensors integrating an infrared thermal sensor and a non-metric visual sensor, enabling the capture of RGB and TIR images simultaneously from the same point of view [44]. Working with this type of low-cost equipment requires careful work during the planning and execution of the activities in order to overcome their limitations and obtain accurate results [45]. To capture the images, the flight plan must be designed to adhere to the requirements of the model's objective and the specifications of the platform and low-cost dual sensor to be used. The whole set of flight parameters to be defined must take into account the resolution, sensitivity, and contrast limitations of the most limiting equipment, generally the thermal sensor [46]. The flight design is usually performed by specific software automatically by inserting the initial flight parameters [47] described below. For the capture of the photogrammetric block of large areas such as landfills, it is advisable to use radial flights with flight heights high enough to achieve a ground sample distance (GSD) suitable for the application [48], in this case the identification of thermal anomalies, where the decimeter accuracies are sufficient to set the detection of a possible hot spot. On the other hand, the frontlap and sidelap used for TIR capture set high values with respect to UAV photogrammetry for visual images, proposing values higher than 70% [49] or even more in both cases [50]. The last parameter to consider is the flight speed, which determines the total flight time, a factor to be taken into account due to UAV autonomy limitations. It is set below 3 m/s to avoid resolution, sharpness, and contrast issues associated with TIR images [51]. In order to obtain an accurate thermal model, which also enables the correct quasi-automatic identification of thermal anomalies, the planning also has to take into account the surface emissivity, the sensor stabilization, and the atmospheric conditions of the day of flight. The emissivity of the surface materials will depend on the nature of the MSW, so a suitable value needs to be adapted for the characterization of the entire landfill. Due to the instability of low-cost infrared sensors, it is recommended to set a sensor stabilization time of about 30 min before taking flight [52; 53]. Finally, atmospheric and climatological conditions determine relevant parameters such as the ambient temperature, wind, solar radiation, and cloudiness. When scheduling the flight, in order to minimize the effect of solar radiation and maximize the temperature difference between the hot spot and the rest of the landfill, the summer season should be avoided [1; 4], and flying in the early hours of the day before sunrise or after sunset is preferable [13]. Finally, in order to correctly georeference the RGB and TIR images, ground control points (GCPs), which are one of the most influential parameters in the final accuracy of the model [54; 55], are used in a standardized manner. For this purpose, metallic plates can be used, as they are visible in both types of images. These plates must be distributed homogeneously along the landfill to be represented [56; 57]. In addition, the use of thermal control points (TCPs) is proposed, which enables the contrasting of the temperature measured with the UAV sensor and the surface temperature in order to thermally calibrate the final model, adjusting it more closely to reality [58].