Positioning today is performed almost exclusively by means of the satellite positioning
system GPS. For this type of hydrographic positioning, the so-called C/A-Code (Civilian Access Code) with code correction in the metre range is usually sufficient. The correction signals of the International Association of Lighthouse Authorities (IALA) are normally used and broadcast in the radiobeacon band. The accuracy is generally on the order of ± 2 m to ± 10 m. If a higher precision of positions is required in individual cases, own reference stations or the Satellite Positioning Service of the German State
Survey (SAPOS) are used. The International Hydrographic Organization (IHO) requires an accuracy between ± 5 m and ± 8 m for positioning, depending on water depth (0 to 60 m), with a confidence level of 95 %.
Depth measurements are normally made using single-beam echosounders
with a frequency range of 210 kHz, which are accurate to 5 – 20 cm depending on the bottom characteristics and depth. With muddy sea bottoms, we additionally use 15 or 30 KHz transponders. Single-beam echosounders only scan the seabed directly below the survey vessel. To obtain an accurate record of the morphological seabed features, the sounding lines should be perpendicular to the depth contours. The closer the spacing between the sounding lines, the more details on the seafloor can be mapped.
In some areas, it will be necessary to achieve complete coverage. An example
is the Kadetrinne in the Baltic Sea between Darss and Gedser. It is a highly congested narrow channel with numerous shoals and obstructions. In such cases, multi-beam echosounding equipment is used which achieve a seabed coverage that is several times the water depth. The depth measurements in such areas should normally have an accuracy better than ±5.. 0.9 m, depending on water depth and taking into account all error influences (IHO, 1998, p. 5).
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