## 3 Results
In order to determine the spatial and temporal variability of seafloor relief of the waterbody, only those pairs of points were compared (with the same rectangular coordinates on both DTMs) for which normal heights could be calculated. There were 21322 such pairs of points in the area under study. In the next stage, a DTM model was created (again using the triangulation model) showing differences in normal heights between points with the same rectangular coordinates (X\({}_{\text{2000}}\), Y\({}_{\text{2000}}\)). The following formula was used to calculate \(\Delta\)H\({}_{\text{2018-2016}}\):  
\[\Delta H_{\text{2018-2016}}=H_{\text{2018}}-H_{\text{2016}}, \tag{6}\]  
Figure 5: Illustration of point height determination in a Triangulated Irregular Networks (TIN) model. where:  
H\({}_{2016}\), H\({}_{2018}\) - normal heights of a point on DTMs based on data acquired by the geodetic method in 2016 and 2018, respectively. After the height differences between DTM models were generated, a tool for creating contour line, available in the TBC software, was used to mark curves at the following depths: \(-60\) cm, \(-40\) cm, \(-20\) cm, \(0\) cm, \(20\) cm, \(40\) cm (Figure 6). The software generated them as smooth curves using the spline method. These data show that the greatest bottom variability in the waterbody adjacent to the municipal beach in Gdynia is observed along the coastline. Differences between normal heights range from -60 to -20 cm (Figure 6). Since the normal heights acquired by the geodetic method in 2016 were subtracted from those measured in 2018, it appears that the coastline moved landwards, which resulted in a decrease in the area of the municipal beach in Gdynia. However, when it comes to the remaining part of the waterbody (not directly adjacent to the coastline), one can see that the variability of the seafloor relief in this area is diversed and ranges from -30 cm to 30 cm, regardless of the distance from the coastline. In order to perform an analysis of bathymetry changes, the waterbody was divided into 12 areas according to height differences every 10 cm (Table 2). Table 2 shows that for nearly 2/3 of the terrain area (64.53%), the height changed only slightly (not more than 20 cm), and for nearly 90%, not more than 30 cm. Changes in the normal heights in the remaining waterbody part (11.3% of the total area) were greater than 30 cm, with the majority of this \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Min. Elevation (m)** & **Max Elevation (m)** & **Real Area (m\({}^{2}\))** & **Percentage of Total Area (\%)** \\ \hline \(-0.605\) & \(-0.600\) & 2.3 & 0.01 \\ \(-0.600\) & \(-0.500\) & 172.7 & 0.82 \\ \(-0.500\) & \(-0.400\) & 709.6 & 3.37 \\ \(-0.400\) & \(-0.300\) & 1227.9 & 5.83 \\ \(-0.300\) & \(-0.200\) & 2317.9 & 11.00 \\ \(-0.200\) & \(-0.100\) & 3182.2 & 15.10 \\ \(-0.100\) & \(0.000\) & 3587.1 & 17.02 \\ \(0.000\) & \(0.100\) & 4511.0 & 21.41 \\ \(0.100\) & \(0.200\) & 3964.3 & 18.81 \\ \(0.200\) & \(0.300\) & 1128.5 & 5.36 \\ \(0.300\) & \(0.400\) & 254.6 & 1.21 \\ \(0.400\) & \(0.438\) & 12.7 & 0.06 \\  & & **S\({}_{\textbf{T}}\) = 21070.7 m\({}^{2}\)** & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Division of the waterbody adjacent to the municipal beach in Gdynia with respect to height differences between 2016 and 2018 Digital Terrain Models (DTM). Figure 6: Height differences between DTMs created on the basis of data acquired by the geodetic method in 2016 and 2018.  
area being situated along the coast. Apart from the division of the waterbody area with respect to height differences between DTMs, an analysis of changes of its volume was also performed (Table 3). Table 3 shows that the sand volume increased by 1695.2 m\({}^{3}\) and decreased by 759.1 m\({}^{3}\) in the area of 21070.7 m\({}^{2}\) (the municipal beach in Gdynia has a sandy bottom) within the past two years.