Zero does not always equal zero. This is particularly true for navigation - specifically for chart zero. This is defined differently for the German North Sea, for example, than for the German Baltic Sea. Denmark, Sweden and Norway, on the other hand, base their depth and height data on yet other reference values, from which the true current water levels can then be derived.
This should be borne in mind by anyone moving from one country to another in a shallow archipelago, for example. Or who wants to sail under a bridge whose clearance is not much greater than the height of their own mast in the sky.
Skipper instructor Leon Schulz has tackled the problem and draws attention to a number of factors that need to be taken into account when calculating the true water depth. His instructions can now be read in the new issue of YACHT (issue 25/2019, available now from newsagents and as a Digital edition available).
