## 1 Introduction  
Maritime delimitation is an international legal concept which denotes establishing a boundary between states. Regarding maritime areas, a state can determine four types of zones: contiguous zone, continental shelf, exclusive economic zone, and territorial sea [1; 2]. Only the territorial sea baseline, which serves as the limit from where the maritime zones are measured, will be analyzed in this paper.  
The territorial sea is part of the sovereign territory of the state. It is a belt of coastal waters extending at 12 Nautical Miles (NM) from the baseline (Figure 1) [3; 4]. In international regulations related to the maritime industry, two types of baselines are found. The first is a normal baseline, defined as:... \"the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state\" [4]. With reference to the Article 5 of the United Nations Convention on the Law of the Sea (UNCLOS), the term \"coastline\" should be understood as the line where land meets the water. Therefore, the normal baseline is coincident to the coastline (low-water line) as portrayed on the charts officially recognized by the states concerned. The fact that this line is almost always under water makes hydrographic surveys necessary for the accurate determination of the low-water line (hence the normal baseline) [5]. This line is used where the coast is regular. The second is a straight baseline which can be applied:... \"In localities where the coastline is deeply indented and cut into, or if there is a fringe of islands along the coast in its immediate vicinity\"... \"Where because of the presence of a delta and other natural conditions the coastline is highly unstable\" [4].  
It is necessary to determine the baseline mathematically in order to establish the outer limit of the territorial sea, which is the state boundary [7; 8; 9]. Its determination mainly affects the establishment of the range of sovereign rights of individual states. In the case of Poland, the baseline determination can be considered in the following issues: legal and measurement-related [10; 11; 12].  
In a legal sense, the outer limit of Poland's territorial sea is established by the domestic legal acts presented in [6]. In view of the dates of these documents (1957-1995) [13] and the variability of the hydrological conditions of the Baltic Sea, it should be stated that Poland currently does not have an actual geographical data set defining the maritime state boundary.  
The other issue concerns the methodology of hydrographic measurements on ultra-shallow waters (of the depth below 1 m), i.e., where the baseline is situated [14]. No international standard defines a method of baseline determination, but minimal quality requirements for such measurements are set out in the International Hydrographic Organization (IHO) S-44 standard [15]. Therefore, for many years, baseline measurements have been performed with tachymetric methods [16], satellite methods [17; 18], and direct measurements conducted by surveyors on sounding profiles in seawater [19; 20]. Over the last dozen or so years, high-resolution satellite images [21; 22], Light Detection and Ranging (LiDAR) systems [23], or even Unmanned Surface Vessels (USV) [24; 25] have been used to determine the course of the territorial sea baseline.  
The purpose of this publication is to analyze the variability of the territorial sea baseline determined twice--in 2016 and 2018--with a geodetic method using a Global Navigation Satellite System (GNSS) receiver on the example of a waterbody adjacent to the municipal beach in Gdynia.