## 3 Results
The landmass balance, i.e., the difference between the total erosion volume (V\({}_{\text{TE}}\)) and the total accretion volume (V\({}_{\text{TA}}\)) shows that the sand volume in the area increased by 936.1 m\({}^{3}\). This amount is relatively small because if the excess sand was spread evenly across the entire area, the bottom of the waterbody adjacent to the municipal beach in Gdynia would increase by just 4 cm. The analyzes also included an assessment of the territorial sea baseline variability within the past two years. The distance between the baseline and the coastline was adopted as the measure of changes of the territorial sea baseline. Since the coastline moved landwards by several meters during the period under analysis, the reference line was determined first, and it was used as the basis to calculate the distance to the baselines. To do this, the coordinates of points measured along the coastline in 2016 and 2018 were used. Subsequently, the data were used to calculate the linear regression coefficients (for the reference line) by the least-squares method from the following formulas [45; 46]:  
\[b=\frac{\sum(X_{\text{C}}\cdot Y_{\text{C}})-N_{\text{C}}\cdot\overline{X}_{ \text{C}}\cdot\overline{Y}_{\text{C}}}{\sum Y_{\text{C}}^{2}-N_{\text{C}}\cdot \overline{Y}_{\text{C}}^{2}}, \tag{7}\]  
\[a=\overline{X}_{\text{C}}-b\cdot\overline{Y}_{\text{C}}, \tag{8}\]  
where:  
\(X_{\text{C}}\), \(Y_{\text{C}}\)--rectangular coordinates PL-2000 of the points measured along the coastline in 2016 and 2018,  
\(N_{\text{C}}\)--the number of points measured along the coastline in 2016 and 2018,  
\(\overline{X}_{\text{C}}\)--arithmetic average for the northing coordinates of points measured along the coastline in 2016 and 2018,  
\(\overline{Y}_{\text{C}}\)--arithmetic average for the easting coordinates of points measured along the coastline in 2016 and 2018. The reference line has the following form:  
\[X_{RL}=b\cdot Y_{RL}+a, \tag{9}\]  
where:  
\(X_{\text{RL}}\), \(Y_{\text{RL}}\) --rectangular coordinates PL-2000 of the points that determine the reference line. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Min. Elevation (m)** & **Max Elevation (m)** & **Erosion Volume (m\({}^{3}\))** & \begin{tabular}{c} **Percentage of Total** \\ **Erosion Volume (\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Accretion Volume** \\ **(m\({}^{3}\))** \\ \end{tabular} &
\begin{tabular}{c} **Percentage of Total** \\ **Accretion Volume (\%)** \\ \end{tabular} \\ \hline \(-\)0.605 & \(-\)0.600 & 0 & 0 & 0 & 0 \\ \(-\)0.600 & \(-\)0.500 & 7.4 & 0.97 & 0 & 0 \\ \(-\)0.500 & \(-\)0.400 & 47.7 & 6.28 & 0 & 0 \\ \(-\)0.400 & \(-\)0.300 & 133.1 & 17.54 & 1.4 & 0.08 \\ \(-\)0.300 & \(-\)0.200 & 211.4 & 27.85 & 39.7 & 2.34 \\ \(-\)0.200 & \(-\)0.100 & 207.8 & 27.38 & 230.3 & 13.58 \\ \(-\)0.100 & 0.000 & 117.1 & 15.43 & 467.4 & 27.57 \\ 0.000 & 0.100 & 34.4 & 4.53 & 578.5 & 34.12 \\ 0.100 & 0.200 & 0.1 & 0.01 & 293.9 & 17.34 \\ 0.200 & 0.300 & 0 & 0 & 75.7 & 4.47 \\ 0.300 & 0.400 & 0 & 0 & 8.2 & 0.48 \\ 0.400 & 0.438 & 0 & 0.2 & 0.01 \\  & \(\mathbf{V_{TE}=759.1\ m^{3}}\) & \(\mathbf{V_{TA}=1695.2\ m^{3}}\) & \(\mathbf{V_{TA}=1695.2\ m^{3}}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Volume changes of the waterbody adjacent to the municipal beach in Gdynia with respect to height differences between the DTMs. After the reference line was determined, the distances between the approximate line and the baselines were calculated. Lines perpendicular to the reference line were drawn; they can be described by the formula:  
\[X_{PL_{i}}=-\frac{1}{b}\cdot Y_{PL_{i}}+a_{i}, \tag{10}\]  
where:  
\(X_{\text{PL}i}\), \(Y_{\text{PL}i}\) - rectangular coordinates PL-2000 of the points that determine the i-th line perpendicular to the reference line,  
i - numbering of perpendicular lines, increasing southwards. The formula (10) does not provide the numerical value of parameter \(\text{a}_{\text{i}}\) because it depends on the distance between successive perpendicular lines.