26 | Berichte des BSH Nr. 57
1 Introduction
The tidal parameters are characteristic values that describe the average be-
haviour of the tide at a location. There are values for the heights as well as for
the times of high and low water.
The tidal parameters are important for a wide range of marine and nautical infor-
mation. For example, the water level forecasts for the North Sea, which are pro-
duced several times a day by the Federal Maritime and Hydrographic Agency
(BSH), are referenced to the Mean High Water or Mean Low Water. Mean tide
intervals are used to indicate the time differences of high and low water between
different tide gauges, e.g. relative to the reference ports in the tide tables. These
tidal intervals are also required for the creation of co-tidal charts, which represent
the temporal course of the tide in the sea. In addition, the Mean Spring Range
and Mean Neap Range are traditionally shown in co-range charts.
The calculation of the tidal parameters, as has been carried out at the BSH
for several decades, is based on the analysis of the semi-monthly inequality.
Inequality refers to the astronomically induced deviation of an individual tidal
value (e.g. a high water height) from the corresponding mean value. The semi-
monthly inequality is the part of the total inequality that depends on the phase
of the moon and thus corresponds to the cycle of spring and neap tides. The
semi-monthly inequality is presented in more detail in section 2. This method is
used to calculate a set of around 30 tidal parameters (see section 3).
The aim of the tidal parameters is to characterise the portion of the astronomi-
Sal tide from the total water level. This is achieved not only by the special analy-
sis procedure, but also by filtering the water level observations used as input
data. Before and during the analysis, large outliers in height and time (caused
Dy storm surges, for example) are removed from the time series, as these are
not characteristic of the astronomical tidal component. Operationally, the validity
of the tidal parameters is determined for an upcoming calendar year. The values
are published, among other sources, in the tide tables and are therefore appli-
cable to a period that may not yet have occurred.
The calculation of the tidal parameters is based directly on the evaluation of
observation data and must not be confused with predicted tidal data. Well-
known methods for tidal analysis and the subsequent tidal prediction are the
widely used ‘harmonic method’ (e.g. Godin, 1972) and the ‘harmonic rep-
resentation of inequalities’ (Boesch & Müller-Navarra, 2019), which is operation-
ally used at the BSH for the German tide gauge locations.
