## 2 Methods
### The Photogrammetric Processing  
The photogrammetric project was carried out with Pix4 Dmapper Pro, version 4.4.9. Only the four multispectral channels were considered, having 6.8 cm GSD; higher resolution RGB imagery was disregarded, as it is recorded in the JPEG format with a high compression factor, and has low quality compared to photogrammetry requirements. The processing followed the usual pipeline [35; 36]: image alignment, tie point extraction, manual measurement of GCPs and CPs, bundle block adjustment (BBA), generation of dense point clouds, digital surface modeling (DSM), and creation of orthomosaic and reflectance maps. The software allows only one set of calibration target images to be used per project, so the photogrammetric processing followed a single-block approach. Four scenarios were depicted based on georeferentiation methodology and radiometric processing:  
1. Direct georeferencing (DG) scenario: no GCPs were inserted in the BBA and each sub-block was processed by direct photogrammetry using positions from the Sequoia integrated GPS receiver. This scenario was used only in geometric assessment. 2. Independent georeferentiation/independent radiometric processing (Ig/Ir) scenario: the two blocks were independently processed in terms of geometry and radiometry. This scenario was used both in geometric and radiometric assessment. 3. Independent georeferentiation/joint radiometric processing (Ig/Ir) scenario: this scenario is a variation of the previous one, in which orientation parameters were computed for each block independently, as in the second scenario, but the two flights were then merged for dense point cloud and reflectance maps generation. This scenario coincides with the so-called \"merge option\" in Pix4 Dmapper software, and it is the recommended procedure for processing photogrammetric  
Figure 2: Unmanned aerial vehicle (UAV) survey framework: (**a**) site location; (**b**) the sub-block compositions. Note: light blue lines represent the flight outlines where the overlapping areas are clearly visible. The one considered in the paper is highlighted in red and includes four ground control points (GCPs), named 6, 7, 8, and 9. GCP locations are reported with green triangles. Coordinate reference system (CRS): WGS84/UTM 32N. Central coordinates (E, N): 506500, 5005600.  
blocks with a large number of images and an overlapping area. It should ensure that radiometric differences caused by a misalignment in the dense point clouds are avoided. Scenario Ig/Jr was used only in radiometric assessment. 4. Joint georeferentiation/independent radiometric processing (Jg/Ir) scenario: the two blocks were jointly orientated, and the obtained exterior orientation parameters were then transferred to a single-block project for generation of dense point clouds and reflectance maps. In this scenario, possible radiometric inconsistencies due to separate computation of interior and exterior orientation parameters are eliminated. This scenario was used in both geometric and radiometric assessment. The bundle block adjustment parameters were set according to the described scenario, since they differ in terms of calibration method and camera optimization. In DG scenario, the calibration method was set to the \"alternative\" option. This choice is recommended when the surveyed area is flat (as in this case) and there is availability of good image geolocation; for the Sequoia sensor, the used geolocation comes from the on-board GPS receiver, even if its quality is low, as discussed before. For camera optimization, external parameters were all re-estimated, while for the internal ones they were adopted from the camera model that is delivered by Sequoia directly into the EXIF (Exchangeable image file) section of each image. As we knew from the Pix4D technical support, the parameters delivered into the EXIF are individually determined for each item at the factory. Their reliability is good, as reported in [11], in which the changes between nominal and optimized camera parameters were as low as 0.01%.