## 2 Materials and Methods
### Extraction and Validation of CET-WV Coastlines  
In order to evaluate the newly developed coastline mapping technique (CET), it was necessary to compare its results with the currently available methods and data for the study site. Therefore, the automated extraction of all coastlines within the study area was done using CET. Then, the extracted coastlines (CET-WV) for the whole study site were compared with two different datasets (Figure 6A):  
1. coastlines from the Topographic map of the Republic of Croatia (1:25,000);
2. coastlines derived from preprocessed multispectral WV-2 image (WV-MS) by a tool developed in [14]. The semi-automatic extraction of coastlines from existing state official topographic maps (A) was performed with _ArcScan_ extension of _ArcGIS 10.1_ software. Automatic derivation of coastlines by an open-source tool (B) was performed in _Erdas Imagine 2018 software_, using the provided open-source tool [70]. This comparison (B) is especially important because it demonstrates how detailed and accurate the developed CET method is in relation to similar existing methods for automated coastline extraction and to the officially available data. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **User-Defined Settings** & **Point Type** & **Total RMSE (cm)** \\ \hline \multirow{4}{*}{Alignment parameters} & Accuracy: High & \multirow{4}{*}{Control points} & \multirow{4}{*}{9.68} \\  & Generic preselection: Yes & & \\ \cline{1-1}  & Reference preselection: & & \\ \cline{1-1}  & Yes & & \\ \cline{1-1}  & Key point limit: 40,000 & & \\ \cline{1-1}  & Tie point limit: 10,000 & & \\ \cline{1-1} Dense point cloud: & Quality: High & & \\ \cline{1-1} reconstruction parameters & Filtering mode: & & \\ \cline{1-1}  & Aggressive & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: User-defined settings used during the creation of VHR models (left) and reported RMSE in control and checkpoints (right). A further validation assessment of CET-WV coastlines was performed within the second chosen validation area, which encompassed the 300 m long segment of the Mrtovnjak island coastline (Figure 6B). This assessment was based on a comparison of the CET-WV-derived coastline with the coastline derived from reference data (CET-UAV) collected within the field UAV photogrammetric survey (Figure 5). CET-UAV and CET-WV coastlines were extracted from datasets collected on different dates. Therefore, the derived models (UAV and WV) used for the extraction of coastlines were harmonized by available tidal data according to the determined tidal difference, which was 9.6 cm [Tidal data for chosen dates (28 October 2016 and 13 September 2019) was provided by the Hydrographic Institute of the Republic of Croatia (c) HHI) from the tidal gauge in Zadar]. The reference dataset (CET-UAV) represents the coastline derived by the CET from the UAV photogrammetry-derived multispectral image (UAV-MS) and the digital surface model (UAV-DSM). Models derived from the UAV survey have a significantly higher spatial resolution (5 cm) than WV-derived models (50 cm for WV-MS and 100 cm for WV-DSM). Thus, the corresponding coastline can be used as reference data for the validation of the accuracy and quality of CET-WV-derived coastlines. Since, in this case, CET is used for coastline extraction from both validated models (WV) and reference models (UAV), this validation rather represents the difference in the accuracy of the models used for coastline extraction (WV vs. UAV). Thus, the conducted validation evaluates the potential of WV-derived models for accurate large-scale coastline mapping. The validation of horizontal accuracy of the CET-WV-derived coastline was performed by _Digital Shoreline Analysis System v. 5.0_ (DSAS), an add-in published by the _United States Geological Survey_ (USGS) for the ESRI ArcGIS v.10 software [71]. The DSAS extension allows the user to define a reference line (baseline), which serves as the basis for projecting transects perpendicular to that baseline (Figure 7).