## 2 Materials and Methods
### Field Acquisition and Processing of UAV Reference Data  
Accuracy validation was performed through a comparison of the coastline derived by the CET from WV-derived models with the coastline derived by the CET from reference data (multispectral UAV photogrammetric survey). Field acquisition of imagery required for the derivation of reference models was done using the UAV photogrammetric survey, carried over the validation area on 13 September 2019.  
A UAV photogrammetric multispectral survey was conducted with a functionally assembled system--MAPS (multispectral aerophotogrammetric system), comprised of _DJI_ _Matrice 600 PRO_ UAV (Figure 4A), _Gremsy T1_ gimbal (Figure 4B), and _MicaSense RedEdge_ _MX_ multispectral camera (Figure 4C-1). Due to the advanced flight capabilities of _DJI_ _Matrice 600_ PRO, the assembled system is capable of conducting complex photogrammetric surveys [67], while, at the same time, the integrated _Gremsy T1_ gimbal ensures constant stabilization and positioning of the multispectral camera. The integrated _MicaSense RedEdge_ _MX_ camera allows the acquisition of multispectral images with the simultaneous collection of five different spectral bands (R, G, B, RedEdge, NIR) [68].  
The spatial extent and parameters of the photogrammetric flight mission were planned and automated within the _Universal Ground Control Software_ 3.3.348 (UgCS) application. The ground sampling distance (GSD) of the planned flight mission was set to 5 cm, forward and side overlap were set to 75%, constant flight height was set to 73.3 m above the ground level, and double-grid flight profiles were chosen during the planning of flight missions (Figure 5). In order to improve the overall georeferencing of created models, nine GCPs were introduced within the validation area, whose precise coordinates were collected using _Stonex S10 RTK-GNSS_ and harmonized according to the available tidal data. A multispectral camera was calibrated using the calibration panel (Figure 4C-2) before and after the conducted flight mission. In total, 2319 multispectral images were collected within the carried survey.  
Figure 4: Different components (**A**,**B**,**C**-**1**,**C**-**2**) functionally integrated into MAPS (**left**); spatial extent of the multispectral UAV photogrammetric survey (**right**).  
Figure 5: Parameters and the spatial extent of the multispectral UAV photogrammetric survey planned over the validation area within UgCS.  
The multispectral imagery was processed using the _Agisoft Metashape 1.5.1_ commercial software, which uses structure-from-motion (SfM) and multiview-stereo technology for the creation of high-detailed centimeter resolution models. The imagery processing methodology was adjusted according to the recommendations given in similar research [69]. Some of the user-defined parameters used for the creation of VHR models in Metashape are given in Table 3. In total, nine GCPs were added to the Metashape, from which five were used in Metashape as control points and four as checkpoints.  
Image processing workflow resulted in the derivation of a single 5-band multispectral image (UAV-MS) and digital surface model (UAV-DSM) of validation area, with a spatial resolution of 5 cm. Then, developed CET was applied for the extraction of the coastline from these created reference UAV-MS and UAV-DSM models. Since the used multispectral camera (MicaSense RedEdge MX) did not have Coastal and NIR2 spectral bends, those were replaced with Green and NIR bands for the calculation of NDVI and NDWI:  
\[NDVI=\frac{NIR-R}{NIR+R} \tag{3}\]  
\[NDWI=\frac{G-NIR}{G+NIR} \tag{4}\]  
Furthermore, user-defined thresholds within the CET had to be adjusted accordingly. The user-defined threshold for the NDVI was set to \(\geq\)0.5, while NDWI and DSM remained identical to default thresholds (\(\leq\)\(-\)0.2 for NDWI and >0 for DSM).