## 4 Discussion  
The purpose of this publication is to analyze the variability of the territorial sea baseline determined twice, in 2016 and 2018, by a geodetic method using a GNSS receiver in the 400 m long waterbody adjacent to the municipal beach in Gdynia. Apart from that, changes of bathymetry in the area under analysis were determined to verify the study results. These measurements have shown considerable spatial and temporal variability of the baseline course along a short section of sandy beach. Figure 9 shows that the territorial sea baseline is very unstable because, in some places, it moved by even 20-25 m, landwards and seawards. It is particularly noticeable in the middle section of the waterbody, between the coastline chainage of 0.1 and 0.2 km. Such large variability of the baseline may have been caused by successive \"shallows\" on several profiles (Figure 10). Between the \"shallows\" there were \"depressions\" whose depths corresponded to the baseline. Therefore, one can suppose that these local bottom surface elevations change periodically, and one can conclude that the reliability of the baseline measurements can decrease quite quickly. 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. 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. 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 measured waterbody is adjacent to the Yacht Basin named Mariusz Zaruski in Gdynia, which are approached by vessels with a draft not exceeding 3.20 m. Due to the fact that fine sand is lying on the bottom in this area, this shipping connection may have an impact on the bathymetry changes. North-east and east winds occurring in autumn and winter are another very important hydrometeorological factor; their forces reach even 3-5 on the Beaufort scale. These winds cause the formation of high waves and significant changes in sea levels during this period, which may be up to 1.37 m higher or 0.97 m lower than average. As for the sea currents, in this region along the breakwater, there is a weak north current, which during northern and north-east winds reaches a speed of up to 2 knots [47]. Hence, according to the Authors, the most important hydrometeorological factor that had an impact on bathymetry changes of the waterbody adjacent to the municipal beach in Gdynia and the territorial sea baseline was wind. The baseline measurements performed by geodetic methods had many limitations, which included low coverage and duration. Therefore, the Authors recommend conducting bathymetric measurements in ultra-shallow waters by the hydrographic method, which involves the use of an unmanned surface vessel (a drone with a dozen centimetre draft) [48; 49; 50; 51; 52; 53; 54; 55; 56], on which a GNSS receiver and a Single Beam Echo Sounder (SBES) can be mounted. These measurement techniques enable high precision (1-5 cm (p = 0.95)) hydroacoustic surveys [57; 58; 59; 60; 61; 62; 63; 64] performed during a relatively short time. However, one must bear in mind that an unmanned surface vessel cannot measure a whole waterbody [24]. It is caused by the following factors: minimum sounding depth (ca. 30 cm) and vessel draft. Hence, the area between the coastline and the 30/40 cm isobath should be measured by the geodetic method [20] or photogrammetric method with unmanned [65] or manned aircraft [66; 67].