## 2 Materials and Methods  
### Planning Measurement Work  
The baseline measurements were carried out in the waterbody adjacent to the municipal beach in Gdynia, which is located in the Bay of Gdansk. This area is characterized by a typical coastline (straight line sandy section), reinforced with tetrapods and concrete wharves. This is an area with small hydomorphological changes because it is located inside the Bay of Gdansk, and in its immediate vicinity there is no river mouth that could cause significant changes in the shape of the seafloor. Furthermore, based on long-term research conducted at 35 gauging stations along the Baltic Sea [26], it appears that it is a non-tidal sea because the differences in water levels due to the impact of the tides  
Figure 1: The effect of the baseline to the outer limits of maritime zones. Own study based on: [6].  
are small. The minimum tide stood at 3.5 cm at the eastern part of the Bay of Gdansk (Baltiysk), near the place where the baseline measurements were made.  
This research was performed on 23.06.2016 between 08:30 and 18:00, as well as on 19.07.2018 between 11:00 and 14:00. Meteorological conditions that prevailed during the measurements were beneficial, i.e., there was no rain, and the wind did not blow. In order to determine them, short-term weather forecasts were used, which are available on the website: [[http://www.pogodynka.pl/](http://www.pogodynka.pl/)]([http://www.pogodynka.pl/](http://www.pogodynka.pl/)).  
Before research started, the sounding profiles were planned, along which the operator performing the measurement with a GNSS receiver mounted on a pole was supposed to move. Forty main sounding profiles were determined perpendicularly to the shape of the coastline, and the distance between them was 10 m (Figure 2) [27]. The profiles were divided into four sections perpendicularly to the coastline because of the non-linear course of the coast. The above described parallel line method is a common practice used in hydrography because it allows coverage of the entire area with the same accuracy [28; 29].  
In order to determine territorial sea baseline (according to the Article 5 of UNCLOS Convention), it is necessary to assess the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state [4]. In geographical areas where the tidal range is negligible (for example less than 30 cm) and in non-tidal areas, such as the waterbody adjacent to the municipal beach in Gdynia, the low-water line should be referred to Mean Sea Level (MSL) or another level as closely equivalent to this as is practical and acceptable to Hydrographic Offices [30]. For the purpose of this publication, the low-water line was acquired from the gauging station in the Gdynia marina, situated 200 meters from the measurement site. The minimum water level at the site amounted to 415 cm (on 04.11.1979) and it was referred to the sea level in Kronstadt.  
The baseline depth at the measurement time is determined by recording the water level readings. For this purpose, the Institute of Meteorology and Water Management (IMGW-PIB) weather service was used, which provides information on the water levels based on mareograph readings. Under the regulation [31], water levels in Poland can be referred to the Kronstadt vertical coordinate system (PL-KRON86-NH), until 2019, and then to Amsterdam (PL-EVRF2007-NH). The required water level can be determined using the height transformation between the systems PL-EVRF2007-NH and PL-KRON86-NH [32; 33]:  
\[H_{PL-KRON86-NH}=H_{PL-EVRF2007-NH}-dH, \tag{1}\]  
where:  
HPL-KRON86-NH--normal height of a point in the PL-KRON86-NH system [m],  
Figure 2: The planning of hydrographic profiles during baseline measurements in Gdynia.  
HPL-EVRF2007-NH--normal height of a point in the PL-EVRF2007-NH system [m], dH--difference of normal heights between the systems PL-EVRF2007-NH and PL-KRON86-NH, which depends on the geodetic latitude (\(\phi\)) and longitude (\(\lambda\)) of a point [m].