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 All sounding depths depend on the particular
water level at the time of measurement. The purpose of sounding reduction is to reduce the measured depths to a reference plane, the chart datum. With respect to the Baltic Sea, Schleswig-Holstein uses the ordnance datum as reference plane, Mecklenburg-Vorpommern the long-term mean water level. By means of gauge measurements, the difference between the current water level and the mean water level is computed. Thus, to obtain correct depths, gauge readings are immediately
reduced to the reference plane. In the North Sea, the ordnance datum is not suitable as a reference plane for navigational purposes because water levels during low tide are normally below it. Therefore, the chart datum used in the North Sea so far is the mean low water springs (MLWS). As of 2005 MLWS is being replaced by Lowest Astronomical Tide (LAT). Navigating officers can then compare the charted depths with the draught of their vessel.
Tidal reductions for the North Sea are made using cotidal charts and gauge
readings.The cotidal charts indicate with respect to each location:
- the time it takes for the tidal wave to travel from the gauge position to the sounding position
- difference between the springs tidal range at the gauge position and sounding position.
For research purposes and in coastal engineering, the evaluated depths are also referred to chart datum but with a slighly reduced accuracy.
Example of cotidal charts
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