## 3 Results
It was assumed for this study that the distance will be 1 m.  
The distances between the reference line and the baseline (\(\text{d}_{\text{i}}\)) were calculated from the coordinates of these lines intersecting with the perpendicular line drawn to the reference line (Figure 7):  
\[d_{i}=\sqrt{\left(X_{RL_{i}}-X_{TSB_{i}}\right)^{2}+\left(Y_{RL_{i}}-Y_{TSB_{i }}\right)^{2}}, \tag{11}\]  
where:  
\(X_{\text{RL}i}\), \(Y_{\text{RL}i}\)--rectangular coordinates PL-2000 of the reference line intersection points with the i-th line perpendicular to it,  
\(X_{\text{TSBi}}\), \(Y_{\text{TSBi}}\)--rectangular coordinates PL-2000 of the baseline intersection points with the i-th line perpendicular to the reference line. If the perpendicular line intersected the baseline at more than one point (Figure 8), the coordinates of these lines intersection were averaged first (\(\overline{X}_{TSB_{i}}\), \(\overline{Y}_{TSB_{i}}\)):  
\[\overline{X}_{TSB_{i}}=\frac{\sum\limits_{j=1}^{k}X_{TSB_{i,j}}}{k}, \tag{12}\]  
\[\overline{Y}_{TSB_{i}}=\frac{\sum\limits_{j=1}^{k}Y_{TSB_{i,j}}}{k}, \tag{13}\]  
Figure 7: Graphical method to determine the distance between the reference line and the baseline. where:  
j--number of the baseline intersection with the i-th line perpendicular to the reference line,  
k--the number of the baseline intersections with the i-th line perpendicular to the reference line,  
and subsequently they were used to calculate the distance to the reference line:  
\[d_{i}=\sqrt{\left(X_{RL_{i}}-\overline{X}_{TSB_{i}}\right)^{2}+\left(Y_{RL_{i}}- \overline{Y}_{TSB_{i}}\right)^{2}}, \tag{14}\]  
After calculating the distance between the reference line and the baselines (measured in 2016 and 2018), the spatial and temporal variability of the territorial sea baseline were determined by calculating the distance between the baselines (\(\Delta\)d\({}_{2018\text{-}2016i}\)) using the following formula:  
\[\Delta d_{2018\text{-}2016_{i}}=d_{2018_{i}}-d_{2016_{i}}, \tag{15}\]  
where:  
d\({}_{2016i}\)--distance between the baseline measured in 2016 and the reference line calculated along the i-th line perpendicular to the reference line,  
d\({}_{2018}\)--distance between the baseline measured in 2018 and the reference line calculated along the i-th line perpendicular to the reference line. In Figure 9, there is presented the changes in the territorial sea baseline course in the waterbody adjacent to the municipal beach in Gdynia over two years. Figure 9 shows that the spatial and temporal variability of the baseline course is quite large. The largest differences were observed in the middle section of the waterbody, where the territorial sea baseline moved landwards by several meters (coastline chainage: 0.125-0.175 km), and even moved by more than twenty meters into the sea (coastline chainage: 0.2 km). Moreover, considerable differences were noted in the southern part of the waterbody (coastline chainage: 0.325 km) between the baselines, from several to even 25 m.  
Figure 8: A specific case, in which the line perpendicular to the reference line intersects the baseline at more than one point. Statistically, the baseline has changed with a mean variability of 9.50 m over the two-year period of observation. Thus, it can be claimed that the position of the territorial sea baseline along the waterbody is not stable. This statistical measure was calculated from the following formula:  
\[\sigma_{\Delta d_{2018-2016}}=\sqrt{\frac{\sum\limits_{i=1}^{N}\left(\Delta d_{2 018-2016},-\Delta\overline{d}_{2018-2016}\right)^{2}}{N-1}}, \tag{16}\]  
where:  
N--the number of lines perpendicular to the reference line. At the final stage of the work, it was decided to visualize the course of the baselines, which were determined in 2016 and 2018 (Figure 10). To this end, free Google Earth Pro software was used to map the Earth by superimposing satellite images, aerial photography, and Geographic Information System (GIS) data onto a 3D globe.