Licensing of harmonic constants data.
Tidal prediction programs rely on the accuracy of harmonic constants data. Early
in 2001 the UK Hydrographic Office asserted its harmonic constants were
protected by Crown Copyright, so their use requires payment of a license
fee. It is Government policy to make the UKHO a trading agency, ie profit
making.
Before that, many tidal prediction programers had believed harmonic
constants data was free. In the USA such data is free under US Freedom
of Information legislation, but so far as other countries' harmonic
constants are concerned, programers now realise they need permission,
and in the case of the UKHO data, they know they will only get permission
at a price.
The main non-commercial source of tidal information is XTide and its
derivatives, WXTide32 for Windows and XTide on the web. Following the
UK HO announcement, David Flater the originator of XTide, took the precaution
of removing "all of the data originating with the International Hydrographic
Office or the Table des Marées des Grands Ports du Monde" from
his products. The XTide website, run by Dean Pentcheff, did the same
when requested to by the UK HO. Anyone who downloaded XTide or WXTide32
for Windows before the UKHO's action will have an unlicensed dataset
of harmonic constants, which may work, but the reliability of which
must be in question. There are dozens of other programs for different
operating systems including PSION and Palm organisers. Whatever the
operating system, the best advice today must be to use licensed data
- see above for sources. Typically predictions for the next few days
are free over the web but data for longer periods has to be purchased.
Matters did not end with the withdrawal of UKHO data. In November
2003 the British Oceanographic Data Centre (BODC) based at the Proudman
Oceanographic Laboratory in Liverpool granted David Flater limited permission
to use its harmonic constants in his free programs in order "to
see the UK back on the map". These predictions are not the same
as those computed by the Proudman Oceanographic Laboratory. The Flaterco
predictions for North Shields are available to anyone free of direct
charge.
All tidal prediction programs are based on calculations and cannot
take into account local weather conditions. Actual heights in river
estuaries like North Shields can differ from predicted heights if, for
example, there has been heavy rain over the past few days in the river
catchment area.
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