## 5 Hydrographic Depth Measurement  
Bathymetric measurements are performed where the territorial sea baseline is situated, i.e., in coastal zones, which are special waterbodies from a hydrographical perspective. Their characteristic features include very small depths (less than 1 m) and high diversity of the seafloor relief. These prevailing conditions can significantly hinder or even prevent hydrographic work. Performing classical hydrographic measurements with manned vessels might prove impossible due to the excessively large draft of the vessels and echo sounder transducers installed on their bows. Therefore, according to the authors it seems optimal to use USVs for baseline measurements [31; 32; 33; 34; 35]. An alternative method is the use of manned or unmanned aerial vehicles equipped with LiDAR to determine the baseline. It should be remembered that this method has limited application, because it depends on the water quality classification at the place of measurement [12; 13]. This section will provide an analysis of USV equipment, which will perform the planned measurement. The technical equipment used territorial sea baseline measurements (necessary on an unmanned vessel) includes [10]:  
* a miniature SBES
* a GNSS receiver using a GNSS geodetic network or a DGPS receiver. Currently, a miniature MBES or SBES is the only hydroacoustic device intended for depth measurements that can be installed on an USV. Other devices, such as a MBES or a MTES (multiple transducer echo sounder) cannot be placed on a vessel because of the considerable size and weight of the transmitting elements. The working frequency is the main factor taken into account when choosing an echo sounder. Therefore, echo sounders operating at high frequencies should be used in bathymetric measurements in ultra-shallow waters. The other (less important) requirements for SBESs in baseline measurements are described in detail in [10]. Before starting the bathymetric measurements, the measuring equipment must be calibrated to verify its operation and to eliminate errors. Therefore, two actions must always be performed when using an echo sounder [37; 38]:  
* measurement of the vertical distribution of the speed of sound in water
* measurement of the draft of the echo sounder transducer. Determination of the speed of sound in water is described in detail in the previous section on oceanographic measurements. However, one must make sure that this velocity is set in the echo sounder. This can be done with the free software SonarMite App+, which can be downloaded and installed on a mobile device (laptop, tablet, smartphone). First, the echo sounder must be coupled with the mobile device by short-range wireless data transmission (Bluetooth). Subsequently, SV+ and SV-buttons can be used to set the calibration velocity in the echo sounder. The SonarMite App+ program enables displaying the results of depth measurement on the mobile device screen (Figure 9). The draft of an echo sounder is another component of the depth measurement. A tape measure can be used to determine this since the draft of an echo sounder transducer installed on an USV is small (ca. 10 cm), This device will enable estimation of the transducer draft under the water surface with an accuracy of approximately 1 cm. Since it is a constant value, it must be added to the depth measured by the echo sounder during data processing. Another measurement-related issue is the precision positioning of hydrographic measurements. Two types of receivers, which meet the requirements laid down in the IHO S-44 standard (IHO, 2008), can be used for this purpose, including:  
* a GNSS receiver using a GNSS geodetic network
* a DGPS receiver. The first of these is able to use all global navigation satellite systems (GPS, GLONASS, BDS, Galileo), satellite-based augmentation systems (SBAS) and GNSS geodetic networks. Hence, the average number of satellites available to a carrier phase receiver usually ranges from 16 to 20, which ensures a hydroacoustic device positioning accuracy of 1-2 cm (RMS).