## 5 Hydrographic Depth Measurement
These values can be obtained only with GNSS geodetic networks, which offer paid or free services [39; 40; 41; 42; 43]. Network users can perform GNSS measurements with one of the two measurement methods. The first involves post-processing of observations recorded during a measurement session. The other includes real-time positioning methods, in which position coordinates are determined on an ongoing basis by a GNSS receiver. For performing hydrographic measurements, we are only interested in real-time positioning, i.e., the RTK (real time kinematic) and the increasingly common RTN (real time network) technique. The RTN method involves the generation of correction data based on observations not from one reference station, as in the RTK technique, but from at least several. Its advantages include the fact that the position error does not increase with the distance from the station (even up to 70-80 km) and the results are not inferior to classical RTK measurements. Based on recent stationary measurements [43], it was found that reference stations at larger than dedicated (60-70 km) distances from the user performing the measurements, do not have any practical impact on the positioning error. For example, the error in the horizontal plane for the SmartNet network was 0.8 cm, with the mean distance between the reference station and the user being approximately 40 km. On the other hand, the error for the VRSNet.pl was 2.2 cm, but the mean distance between  
Figure 9: The application window for setting the calibration velocity of a single beam echo sounder (SBES) displaying the results of depth measurements. the reference station and the measuring site was nearly three times greater (ca. 120 km) than with the SmartNet network (Figure 10b). Thus, it can be stated that the difference in the positioning accuracy achieved in both geodetic networks is small. It is recommended that measurements performed by the RTK technique should use corrections transmitted by the reference station, which is the nearest to the measuring site. On the other hand, the operating range should be taken into consideration when choosing a GNSS geodetic network in the RTN technique. It can be noted that only two out of four commercial GNSS geodetic networks (SmartNet, TPI NETpro) provide coverage of the Bay of Gdansk (where the hydrographic work will be conducted) by the network solution (Figure 10b) [44]. The DGPS system can be used as an alternative to GNSS geodetic networks. It is still, in many places, the basic positioning system used in hydrographic measurements [45; 46; 47; 48]. Officially, the DGPS system must ensure a measurement precision (\(p\) = 0.95) of up to 10 m in a horizontal plane in accordance with a standard issued by the International Association of Lighthouse Authorities (IALA) [49]. However, in reality, DGPS users equipped with code receivers can position with an accuracy considerably exceeding the autonomous GPS positioning (5-10 m) [50]. During a recent measurement campaign, the positioning accuracy of the DGPS system in the Bay of Gdansk was determined with respect to its application in hydrographic work. To this end, 4 h of dynamic measurements were conducted, during which approximately 11,500 determinations of position coordinates were performed (Figure 11). An analysis of the study results showed that R95(2D) and R95(3D) measures in the DGPS system were 1.42 m and 2.07 m, respectively. A comparison of the results with the minimum requirements for hydrographic measurements [9] shows that the DGPS system, like GNSS geodetic networks, meets the positioning accuracy requirements regarding all categories of work [47]. Figure 10: The measuring site (**a**) and the borders of a polygon which marks the operating range for individual GNSS geodetic networks in the province of Pomerania (**b**). After the measuring equipment is completed, it can be mounted on an USV. It must be noted that a DGPS/GNSS receiver and a SBES should be fit for integration with a hydrographic vessel.