How Accurate Are The Tide Tables?
Yachtsmen sometimes query the accuracy of tide tables, especially where different sources are involved.
In the UK there are two respected sources of tidal data: The UK Hydrographic Office at Taunton and the Proudman Oceanographic Laboratory in Liverpool. Their tide tables for Standard Ports rarely differ by more than about 0.2m in height of tide and 3 to 4 minutes in time. Differences for Secondary Ports may be a little greater.
At which point it is worth stressing that tide tables do not enjoy Biblical status – they are predictions or forecasts of the heights and times of tides to be expected, based on complex calculations. Mariners may introduce their own pragmatic approach to what is on offer. What degree of accuracy do I require in order to operate my craft safely and efficiently? If navigating through rocks, is it enough to say that in the top half of the tide those rocks will be covered by sufficient water? Or is greater precision required?
Yachtsmen with different levels of experience usually adopt more or less stringent standards. For example a newly qualified Day skipper fresh out of night school has been taught to calculate tidal answers to two or more places of decimals. A more experienced yachtsman may do little more than a few mental calculations to the nearest five minutes and 0.5 metres, because his long experience has taught him what is safe or marginal in a variety of different circumstances.
Furthermore external factors must be considered. Unusually high barometric pressure or persistent winds from a certain direction can significantly affect the depth of water. We allow for these conditions, just as we round off our calculations by applying an arbitrary but safe under-keel clearance.
If tidal calculations for the same port are based on different sources, they may differ noticeably. There is only one answer: Re-check your figures. If no obvious error is detected, opt for the ‘worst scenario’ answer, ie that which predicts the lesser depth of water. Tide times are rarely as critical as heights.
Neville Featherstone is the Editor of the Wiley Nautical Almanac.
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