## 3 Water Level Measurement  
Considering the dynamic changes in sea water levels caused by anemobaric and hydrological factors and tidal elements, obtaining information on the current water level in the area where bathymetric measurements are performed is one of the hydrographer's most important tasks. This information allows the measured depths to be expressed relative to the reference level, referred to as chart datum. For navigation safety, the majority of national hydrographic offices and the IHO have adopted the lowest astronomical tide (LAT) as zero datum [20]. The issue of water level in Poland is managed by Maritime Branches of the Institute of Meteorology and Water Management (IMGW), which is responsible for registration, sharing and archiving information about water levels. They use gauging stations for the purpose, which are situated in marinas and ports. Each gauging station consists of a device for observing water level fluctuations (staff gauge) or a device for observing and registering measurement data (mareograph) and benchmarks, these are referenced to the geodetic height system. The staff gauge is widely used and is the simplest gauging station. A mareograph, which allows for continuous registration of water level changes, is another commonly used device. Automatic telemetric hydrological stations (ATSH) are increasingly often used to record water levels; they are fitted out with water level sensors and they are capable of data sending (GPRS or radio modem) to the IMGW databases. Owing to these functions, the results can be shared automatically in the ITC system ([[http://www.pogodynka.pl/](http://www.pogodynka.pl/)]([http://www.pogodynka.pl/](http://www.pogodynka.pl/))). Notably, all these methods enable reading the water level with an accuracy of 1 cm, although both mareographs and staff gauges are increasingly being replaced by ATSH stations. According to the definition in Article 5 of the UNCLOS Convention, before the determination of the territorial sea baseline is started, it is necessary to determine the lowest water level ever recorded along the coast [5]. Notably, no international legal act regulating the principles of hydrographic work specifies a method for acquiring data on the water level. However, it is possible to obtain such information from the gauging station situated nearest the site of the hydrographic measurements. Therefore, information on the lowest water levels ever recorded at gauging stations in the years 1945-2015 was acquired for this paper from the Maritime Branch of the IMGW in Gdynia (Figure 6). Figure 5: Applications used to plan hydrographic survey, such as AutoDron (**a**) [19] and QINSy (**b**). The zero ordinate for each gauging station is referenced to the water level at Kronstadt, Russia. In Poland, it is \(-508\) cm, except at Darlowo (\(-507.3\) cm). With the lowest and the current water level, e.g., from the IMGW-PIB weather service (Figure 7), one can calculate the depth at which the baseline was situated at the time of measurement (\(d_{TSB}\)) from the following formula:  
\[d_{TSB}=H_{\text{CWL}}-H_{\text{LWL}} \tag{2}\]  
where,  
\(H_{\text{CWL}}\) is the current water level in the adopted reference frame [m],  
\(H_{\text{LW}}\) is the lowest water level in the adopted reference frame [m]. However, the water levels must be reduced to a uniform reference frame valid in a given state [21]. In Poland according to the current regulations [22], water levels are referenced to the height system of Kronstadt (PL-KRON86-NH), (up to 2019) and Amsterdam (PL-EVRF2007-NH). Based on long-term research conducted at 35 gauging stations along the Baltic Sea [23], it appears that the sea is non-tidal because the differences in water level due to the impact of the tides are small. The minimum tide was at 3.5 cm in the eastern part of the Bay of Gdarsk (Baltiysk, Russia), whereas the maximum tides were recorded at the gauging stations located in the Gulf of Finland (Gorniy Institute, Kronstadt, Russia), and amounted to approximately 18 cm.