## 2 Materials and Methods
### Multi-Criteria Decision Analysis (MCDA)
#### 2.2.1 Site Mapping Using UAV  
Each site was surveyed using a tactical-grade Unmanned Aerial Vehicle (UAV), the SenseFly eBee X RTK, which is able to perform direct in-flight georeferencing using two-frequency Real-Time Kinematic (RTK) positioning. For every site, the following products were generated, using PIX4D Mapping Software version 4.6.4: (a) a 2D orthomosaic, (b) a high-resolution Digital Terrain Model (DTM), (c) a high-resolution Digital Surface Model (DSM), (d) Contour Maps, (e) a Photorealistic 3D model. Digital elevation models are commonly used as a data source for spatial analysis because they represent the constantly changing topographic surface [26]. These products aim to obtain slope, aspect, and land cover data and enable LOS visibility and exploratory analysis. The main purpose of the UAV mapping was the creation and validation of models to enable the determination of the final locations of GNSS CORS and CRs. The derivation of the required products was based on the following steps:  
UAV flight planning was carried out using the eMotion Software version 3.12.0 (see Figure 5). During this step, weather conditions, altitude, possible take-off and landing areas have been checked. Additionally, a block with photo overlaps and sidelaps was planned in order to have an adequate overlap between images (latitude 75%, longitude 85%) [27]. Moreover, the total time of flight, the direction of the UAV, and the battery charging levels were estimated to simulate the entire process prior to moving outdoors.  
Figure 5: Flight Planning using the eMotion Software.  
Consequently, the CyCLOPS team proceeded with the airborne mapping of the candidate areas in order to create high-resolution georeferenced 2D and 3D products. The fixed-wing eBee X was equipped with a GNSS receiver capable of measuring with the RTK method, achieving a centimetre-level accuracy without using ground control points and covering areas up to 500 hectares (5 Km\({}^{2}\)) in 90 min of flight. The UAV is equipped with senseFly SODA 3D mapping camera (sensor1\", RGB resolution 5472 \(\times\) 3648 px (3:2)), which has the ability to change orientation during flight to acquire three images (one nadir, and two obliques) each time [28], as shown in Figure 6. In this sense, it obtains a much wider field of view, ensuring excellent 3D mapping results in vertically focused environments; thus, it is suitable for large-scale aerial mapping.  
The point cloud, which was generated using the imported images in Pix4D software, is a set of 3D points modelling reconstruction. The densified point cloud is computed based on the automatic tie points (ATPs) of the initial processing, which contains the following: (a) matching image pairs that allow the selection of which pair of the images are combined and (b) matching strategy for the determination of how the images are matched. Thus, it provides a very accurate background for distance, surface, and volume measurements. Note that the primary products, which have been created through the airborne mapping process are DSM, DTM, and Orthomosaics with a spatial resolution of 2.50 cm/pixel, respectively. These products provide the required criteria via a GIS-based process.