## 3 Results  
During measurement, data processing systems were used: Gauss-Kruger projection, Cartesian coordinate system 2000, height system Kronstadt 86, and the quasigeoid model PL-geoid2011 (Table 1). Digital Terrain Models (DTM) were developed based on 2016 and 2018 measurement data as Triangulated Irregular Networks (TIN) (Figure 4). A TIN model is created as a result of the triangulation of depth points meeting the Delaunay condition, according to which, a circle determined by three vertices of a triangle cannot contain other depth points, with the exception of degeneracy [37]. Triangle vertices are points of known coordinates. Delaunay triangulation is the dual graph of the Voronoi diagram which has the following property: the distance between each point situated inside a polygon and the nodal point, which is the triangle vertex in the next processing stage, is the shortest [38; 39; 40]. It should be remembered that in the TIN method, the high-resolution of the generated DTM model is extremely important. As shown by other studies, the TIN method is more accurate only when the points that were used to generate the DTM model are more concentrated [41; 42; 43]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Country & Poland \\ System/zone & 2000/18 \\ Reference ellipsoid & WGS 84 \\ Semi-major axis of ellipsoid & 6378137 \\ Flattening of ellipsoid & 0.00335281067183 \\ Projection & Gauss-Kruger \\ Latitude of origin & 0 \\ Central meridian & 18 \\ False Northing & 0 \\ False Easting & 6 500 000 \\ Scale factor & 0.999923 \\ Azimuth & North \\ Grid orientation & Rising northeast \\ Height transformation & Geoid \\ Geoid model & PL-geoid-2011 \\ Reference frame & Kronstadt \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Processing parameters during territorial sea baseline measurements in the waterbody adjacent to the municipal beach in Gdynia. Figure 4: Digital Terrain Models (DTM) of the waterbody adjacent to the municipal beach in Gdynia acquired by the geodetic method in 2016 (**a**) and 2018 (**b**). In order to analyze changes in bathymetry, a height grid with the basic square of 1 \(\times\) 1 m (maximum available resolution) was created based on DTM models. It can be used to compare the normal height (H) of a point during the past two years. The height grid was created using a tool for creating surfaces and height grid, which is available in the TBC software. This function uses the generated DTM model as the base for height interpolation of the point P in the terrain surface, whose projection P' on a horizontal plane lies within triangle A'B'C' (Figure 5). The height of the point P is calculated from the formula [44]:  
\[H_{P}=\frac{H_{A}\cdot S_{A}+H_{B}\cdot S_{B}+H_{C}\cdot S_{C}}{S_{A}+S_{B}+S_{ C}}, \tag{3}\]  
where:  
H\({}_{A}\), H\({}_{B}\), H\({}_{C}\) - normal heights of the triangle ABC,  
S\({}_{A}\), S\({}_{B}\), S\({}_{C}\) - areas of the opposite triangles, formed by division of the triangle A'B'C' with line segments connecting the triangle vertices with point P'. [PERSON]'s formula was used to determine the triangle areas (S\({}_{A}\), S\({}_{B}\), S\({}_{C}\)). It is used to calculate a triangle area when the lengths of its sides are known. Since the vertex coordinates for the ABC triangle and the rectangular coordinates for point P are given, they can be used to calculate the areas of component triangles. [PERSON]'s formula can be noted in the following manner:  
\[p=\frac{a+b+c}{2}, \tag{4}\]  
\[S=\sqrt{p\cdot(p-a)\cdot(p-b)\cdot(p-c)}, \tag{5}\]  
where:  
p - semi-perimeter of the A'B'C' triangle,  
a, b, c - lengths of sides of the A'B'C' triangle. Creating the height grid resulted in 25244 points for the 2016 model and 21561 points for the 2018 model, which were further used to calculate normal height differences at the same points (\(\Delta\)H\({}_{\text{2018-2016}}\)).