## 2 Materials and Methods
### Data Collection and Preprocessing
#### 2.2.1 Coastline Information  
Several global and European-level datasets are available depicting coastline information, including the Global Self-consistent, Hierarchical, High-Resolution Geography Database (GSHHG) [45], the USGS Global Shoreline Vector and the European Environment's Agency Polygon Coastline Dataset for Analysis. (EEA). GSHHG is a global dataset that combines information on shorelines from two geographical databases, namely the World Vector Shorelines produced by N.O.A.A regarding shoreline data and the CIA World Data Bank II for lake and river boundaries, as well as political boundaries [45]. Its information is organized around four discrete levels: boundaries between land and ocean (L1), lake and land (L2), island-in-lake and lake (L3), and pond-in-island and island (L4) [45]. The USGS Global Shoreline Vector developed by the United States Geological Survey (USGS) is a 30 m spatial resolution dataset developed by using annual composites of Landsat satellite imagery from 2014 [46]. The classification of the imagery was performed in a semi-automated way through the manual selection of the training sites representing water or non-water bodies along the coastline [46]. The result of this process was the classification into two three distinct classes, namely continental mainland polygons (5 entries), islands smaller than 1 km\({}^{2}\) (318,868 entries), and islands larger than 1 km\({}^{2}\) (21,818 entries). Finally, the European Environment Agency (EEA) provides a polygon coastline dataset for analysis. These data are a composite product that combines satellite information from two distinct projects, namely, EUHYDRO (Pan-European Hydrographic and Drainage Database, link: [[https://land.copernicus.eu/en/products/eu-hydro](https://land.copernicus.eu/en/products/eu-hydro)]([https://land.copernicus.eu/en/products/eu-hydro](https://land.copernicus.eu/en/products/eu-hydro)), accessed on 6 May 2024) and GSHHG (A Global Self-Consistent, Hierarchical, High-resolution geography database, link: [[https://www.soest.hawaii.edu/pwessel/gshhg/](https://www.soest.hawaii.edu/pwessel/gshhg/)]([https://www.soest.hawaii.edu/pwessel/gshhg/](https://www.soest.hawaii.edu/pwessel/gshhg/)), accessed on 5 May 2024) [47]. The combined dataset was further modified to comply with the requirements and several policy documents, including the EU Nature Directives, Water Framework Directive, etc. [47].  
Although these datasets have been used in similar studies, it was noticed that they present significant deviations from the coastal ground truth--at least in the Mediterranean Sea Coasts (Figure 2). As a result, finer-scale open-source coastline data were retrieved from national geoportals, including Greece, France, Italy, Spain, Malta, and Cyprus, thus creating a hybrid coastline vector dataset combining the different coastline vectors and the EEA coastline file in the case of Slovenia and Croatia. All data were reprojected into a common Coordinate Reference System (i.e., EPSG: 3035-ETRS89/LAEA) and the hybrid coastline was buffered at a 2 km distance to the mainland to create the area of interest for the current research.