An Introduction to Tidal Predictions
- You Get What You Pay For…
The published time and
height of high and low water at any given place is just a prediction - but some
predictions are more accurate than others! Even the best predictions however will be affected by the unknown factors of wind and
atmospheric pressure.
A tidal prediction
comprises many parameters. The
underlying pattern of tidal streams is easy enough to determine from the highly
predictable oscillation of the Earth-Moon-Sun system. Indeed, this astronomical information is sufficient to generate accurate
tidal predictions for the middle of a large body of water such as the Atlantic
or Pacific Ocean, where the tidal range is only a few feet anyway. In coastal
regions however, tidal streams are strongly influenced by the topographic
characteristics of the sea basin, seabed and coastline. The situation is even more complicated in estuaries and rivers.
The quality of predictions for a particular port therefore is improved by the
long-term observation of local tidal patterns. From such surveys are derived Port Harmonic Constants, Shallow Water
Corrections and Seasonal Changes in Mean Level. In the UK this work is carried out by organisations such as the Admiralty
Hydrographic Office, although it is possible for an individual to make
reasonable estimates of the local harmonic constants from around a year’s
worth of locally collected data.
For the Hydrographic Office
and similar organisations there is obvious commercial value in protecting the
intellectual property of their work and commercially published tide tables are
therefore subject to copyright restrictions. Short-term tidal predictions are readily available on the Internet, but
as the reader may have observed, the Hydrographic Office in particular will not
license the electronic reproduction of their tidal prediction data for more than
about one month ahead, although regional oceanographic laboratories sometimes
impose lesser restrictions. It is
useful to emphasise that the intellectual property lies in the harmonic
constants used to generate tidal predictions – it is the quality of this data
that represents the value-add – whereas a very rough approximation of high and low
water can be determined (and published) by anyone who follows the movement of
the Moon across the sky.
Harmonic constant data is
also available in the public domain, generally from academic establishments that
have acquired it for research purposes1. Predictions
generated from this data can provide a useful guide and should be found to have timing errors of little more than ±30 minutes.
Sadly, we are no longer able to present tidal predictions
based on such data as the Hydrographic Office have claimed the rights to this
material and have threatened to prosecute any organisation doing
so whether in the UK or abroad. As a concession, they have made their own EasyTide program
available for free 7-day predictions on the internet at:
http://www.ukho.gov.uk-tideprediction.cfm
It is easy to select and bookmark standard and secondary ports, but the
facility only produces predictions for the following 7 days. The following
link gives predictions for Rochester (Strood Pier):
http://www.ukho.gov.uk-tideprediction.cfmregion=1&locale=England&portid=0109B
As every vessel should carry
reliable tidal predictions for the intended cruising area, the purpose of
placing these predictions on our website was to provide a convenient reference to
assist preliminary cruise planning. Well, that was the idea anyway...
Interestingly,
the Port of London Authority seem unconcerned by the Hydographic Office's dire
legal warnings and their excellent website still contains a full 12 months' set
of tidal predictions for several standard ports in the Thames Estuary. See
our Links page for details. Good on you guys!
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1 The harmonic constant data that we were using is published by the Department of Biological
Sciences, University of South Carolina, USA, to whom it was provided by the
Service Hydrographique et Oceanographique de la Marine, Paris.
Predictions were generated using the X-Tide program running under Unix.
© Nigel Jennings, February 2001
- revised January 2002
