## 2 Materials and Methods
### Development of Coastline Extration Tool (CET)  
The CET is developed within the _ModelBuilder_ extension of _ArcGIS 10.1_ software. The workflow of the CET is very straightforward, as the whole processing process can be divided into main four steps: a _selection of input data and the creation of spectral indices_ (1); _definition of user-defined thresholds_ (2); _extraction of preliminary coastline models_ (3); and _final coastline model aggregation_ (4) (Figure 3).  
In the first step (1), the user defines the input data for CET, where specific spectral bands of multispectral image and corresponding DSM have to be selected. Unlike already existing methods (Table 1), which only use spectral information for the extraction of coastlines, CET uses both spectral and elevation information. While three spectral bands (coastal (C), red (R), and near-infrared 2 (NIR2)) from selected multispectral image (1.1) are used for the calculation of specific spectral indices, the selected DSM (1.2) adds the optional possibility to use elevation information for coastline extraction. Spectral indices used for the extraction of coastline are the Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Water Index (NDWI), as those were already used for successful  
Figure 3: Methodological framework of the automatic coastline extraction tool (CET) development.  
coastal extraction in some previous research [5; 14]. NDVI and NDWI spectral indices were calculated according to the following equations [14]:  
\[NDVI=\frac{NIR2-R}{NIR2+R} \tag{1}\]  
\[NDWI=\frac{C-NIR2}{C+NIR2} \tag{2}\]  
In the second step (2), users can manually adjust the thresholds for the extraction of preliminary coastlines from calculated spectral indices (NDWI and NDVI) and selected DSM. Manual adjustment of thresholds is optional, as users can adjust thresholds in regard to specific characteristics of their study area. For example, if applying CET for extraction of coastline of lakes, which are located at elevations higher than sea level (e.g., 100 m above sea level), the user can adjust the elevation extraction threshold accordingly. If not specified otherwise, default threshold values are used (\(\geq\)\(-\)0.2 for NDVI; \(\leq\)\(-\)0.2 for NDWI and \(>\)0 for DSM) for coastline extraction.  
After the definition of extraction thresholds, two preliminary coastline models are created from spectral indices and the DSM (3). The first preliminary coastline model is extracted from the NDVI and NDWI, while the second one is extracted from the DSM.  
The last step (4) includes the aggregation of two preliminary models into a single final coastline model. Within this step, the user can manually define the minimal level of detection (\(\mathbf{LOD}_{min}\)) for coastline extraction. \(\mathbf{LOD}_{min}\) represents the smallest unit of area that the CET will automatically extract, which should be defined in regards to the spatial resolution of the input data (Figure 3--1.1. and 1.2). The definition of \(\mathbf{LOD}_{min}\) allows the automatic removal of all detected areas that are smaller than a defined value, thus automatically removing most of the areas falsely detected as land (e.g., boats, spectral reflections, etc.). Since the spatial resolution of used WV models was 0.5 m for WV-MS and 1 m for WV-DSM, the value of \(\mathbf{LOD}_{min}\) was set to 5 m\({}^{2}\). However, it should be emphasized that the sizes of the smallest island, islet, rock, or rocks awash within the study area have to be considered before the definition of \(\mathbf{LOD}_{min}\). If the defined value is too high, some smaller islands could be potentially removed.  
The results of the CET tool should always be manually checked and verified. Therefore, all features extracted by CET were visually checked, and all objects that do not represent land were manually removed from the results. Detailed schematic representation of the developed tool is given in Supplementary Figure S1.