## 4 Other Oceanographic Measurements
\begin{array}{ll}\Delta_{STP}=&(S-35)\cdot\left(-1.1244\cdot 10^{-2}\cdot T+7.7711 \cdot 10^{-7}\cdot T^{2}+7.7016\cdot 10^{-5}\cdot P+\right.\\ &\left.+1,2943\cdot 10^{-7}\cdot P^{2}+3.1580\cdot 10^{-8}\cdot P\cdot T+1.5790 \cdot 10^{-9}\cdot P\cdot T^{2}\right)+\\ &\left.+P\cdot\left(-1.8607\cdot 10^{-4}\cdot T+7.4812\cdot 10^{-6}\cdot T^{2}+4.52 83\cdot 10^{-8}\cdot T^{3}\right)+\right.\\ &\left.+P^{2}\cdot\left(-2.5294\cdot 10^{-7}\cdot T+1.8563\cdot 10^{-9}\cdot T^{2} \right)+\right.\\ &\left.-P^{3}\cdot\left(-1.9646\cdot 10^{-10}\cdot T\right)\right.\end{array}  \tag{7}\]  
The accuracy of the formula is estimated at \(\pm 0.3\) m/s within the salinity range of \(0<S<370\%\), temperatures \(4\ ^{\circ}C<T<30\ ^{\circ}C\) and pressures \(1\) kg/cm\({}^{2}<P<1000\) kg/cm\({}^{2}\). However, it is the authors' opinion that speed of sound in water can be determined (in the absence of velocity meters and oceanographic sounds) with oceanographic data shared in the ICT system (as long as it exists) by any institution responsible for monitoring the selected waterbody. One such service is SatBaltyk ([[http://satbaltyk.iopan.gda.pl](http://satbaltyk.iopan.gda.pl)]([http://satbaltyk.iopan.gda.pl](http://satbaltyk.iopan.gda.pl))), which monitors on a continuous basis and publishes information on changes in the Baltic Sea environment on its Internet website. Every six hours each day, it shares high resolution (of 1 km) and high accuracy (of the tenth part) information on the sea surface temperature, water temperature at the following depths: 0, 3, 5, 10, 20, 30 m, and sea surface salinity [28; 29]. Temperature is the most variable factor and has the greatest effect on the speed of sound in water, which increases by 4.5 m/s on average when the temperature increases by 1 \({}^{\circ}\)C. Salinity is another factor that has a moderate effect on the speed of sound. The parameter which describes the salt content in water changes slightly, i.e., its value increases by approx. 1.3 m/s when the salinity increases by 10%. Information about salinity (without having to use measuring equipment) with an accuracy of 1% can be obtained, for example, from the SatBaltyk system. The third factor, with the smallest (slight) effect on the speed of sound in water is the pressure (depth), which increases by approximately 1.6 m/s with a depth increase of 100 m. This has no practical significance in measurements performed in the coastal zone. If one assumes that the gravitational acceleration and water density are constant, then the hydrostatic pressure will depend only on the height of the liquid column. For example, a comparison of the speed of sound in water at a depth of 0 and 1 m shows a small difference of approximately 0.01 m/s [26; 27; 28; 29]. It must be stressed that the accuracy of hydrographic measurements can be significantly affected by short-term (daily) changes in sea surface temperature. Using the information presented in [26], it can be seen that the temperature differences within a 24 h period are caused mainly by the sun warming the water surface (Figure 8). The daily speed of sound changes can reach 40 m/s in summer. Therefore, according to the authors, if measurements are performed with a SBES, the water surface temperature should be measured periodically, e.g., every hour with a thermometer or other equipment  
Figure 8: Daily speed of sound changes in summer in the Bay of Gdánsk. Own study based on [26]. intended for the purpose, as mentioned earlier. The measurements can be used to determine the current calibration velocity of the sounding device. The last oceanographic parameter discussed here is ocean currents, which can be extremely important for navigating a vessel along the established sounding profile. In some situations, it is not possible to perform bathymetric measurements with a USV (because of excessively strong ocean currents in the coastal area).