## 2 Materials and Methods  
### WorldView-2 (WV2) Multispectral Imagery Preprocessing  
Commercial multispectral Standard 2A WV2 imagery was provided to authors through the DigitalGlobe Foundation [54]. Single multispectral (2 m spatial resolution) and panchromatic (0.5 m spatial resolution) WV2 satellite image acquired on October 28 th, 2016, covering the whole Iz-Rava island group, was used as the main data for the extraction and mapping of coastlines. In order to improve geometric, spatial, and radiometric characteristics of given WV-2 images, the following three preprocessing steps were performed: (1) radiometric correction; (2) image pansharpening; and (3) image geometric correction.  
The (1) radiometric correction was based on atmospheric correction performed in Geomatica _Focus 2018_ software, using the _ATCOR ground reflectance workflow_. ATCOR is an atmospheric correction method that reduces various illumination and atmospheric induced effects on satellite imagery [55]. It is often used for the preprocessing of WV imagery [56; 57; 58]. User-defined parameters of ATCOR were adjusted in regard to provided imagery metadata (e.g., sensor type, solar zenith and azimuth, acquisition date, etc.).  
After radiometric correction, multispectral and panchromatic WV-2 images were (2) pansharpened. Currently, there is a large number of pansharpening algorithms, which  
Figure 2: Multispectral Worldview-2 image of study area covering the I2-Rava island group, Northern Dalmatian Archipelago, Croatia.  
differ in terms of fusion quality and induced spectral distortions [59]. One of the most common pansharpening algorithms, which was specially designed for WV imagery, is the hyperspherical color space (HCS) resolution merge algorithm [60], which is implemented within Erdas Imagine 2018 software. This pansharpening algorithm was used for the fusion of multispectral and panchromatic images and the creation of a very high resolution (0.5 m) multispectral image (WV-2 MS) of our study area.  
WV-2 Standard 2A imagery has 5m CE90/LE90 absolute horizontal accuracy specification with 2.3 m RMSE if acquired at less than 30 degrees off NADIR and excluding terrain effects [54; 61; 62]. According to the imagery metadata, the chosen imagery was acquired at 27.5 mean off NADIR angle with coarse DEM terrain correction applied. Therefore, claimed horizontal accuracy should be correct within the whole study area. Nevertheless, such horizontal geolocation accuracy is not sufficient for most applications that require the highest level of accuracy [58]. In order to further improve (3) the geometric accuracy of chosen imagery, several ground control points (GCPs) were introduced. Precise coordinates of GCPs were collected during the field survey with _Stonex S10 RTK-GPS_. The collected coordinates were harmonized according to the available tidal data. In total, 35 different GCPs scattered throughout the whole study area were collected. A pansharpened WV-2 image was geometrically corrected according to the collected GCPs through the rubber sheeting method, represented by a spline transformation in _ArcGIS 10.1_ software [63]. As a result of applied preprocessing, a single very high resolution (0.5 m) multispectral image of the whole study area was created (WV-MS).