Are the tidal predictions reliable ?

All navigation software offer predictions of tides, more or less long range, covering the world's coastlines. Where come these predictions from ? How are they established ? What is their reliability ? These are all questions that any conscientious yachtman is entitled to ask.

SHOM predictions from frbateaux.net website

Origin of tides calculations

Many countries with a coastline have an organization to hydrography, oceanography and marine cartography sometimes. In France, the Service Hydrographique et Océanographique de la Marine performs these tasks. The establishment of tidal predictions represents costs assumed by taxpayers, and thus, results are rarely made freely available to the public. In France, tidal predictions are provided with annual books (SHOM Annuaire des marées, Almanach du Marin Breton) sold to the public or digital files sold to interested agencies (software and nautical guides editors, local administration, aso.).

That's why the majority of navigation software uses the predictions of the free program XTide.

XTide is a program developed for 1998 by David Flater, a member of Free Software Foundation (¹). It uses the calculations of harmonic analysis and prediction of tides provided by NOS (²) U.S.. and files harmonics thereof. These data are provided for a large number of so-called "reference" stations and the calculation makes it possible to extrapolate the deviations for other stations, say "attached". The first interest is that XTide is free, the second is that it can cover mainly coasts around the world.

Tides in applications for iPhone and iPad

The reliability of predictions

Harmonic data provided by XTide take absolutely no account of local conditions that can affect tides actually observed. That's why it comes with no guarantees and disclaim any responsibility for any use that can be done.

The geographical configuration can significantly alter the predictions calculated for a station, and only the long-term observation, by means of appropriate measuring, corrects the calculation to obtain the best accuracy. This is particularly the case of estuaries, or very closed bays. National services such as the SHOM perform locally such measures, wherever the calculations are insufficient.

Nevertheless, ISVs can compare harmonic predictions with local hydrographic services publication, to correct the calculations as close as possible to the local predictions. This is particularly true of Hahn Software who spent much time in recent months to correct the data in its applications AyeTides up to AyeTides XL.

Comparison

Allows a quick comparison chart to gauge differences between the predictions provided by different applications. This is just a "photograph" of a few stations on a single day., with a tidal range given (coefficient 85) and the variations may be more or less important depending on the range. However, it allows to get an idea of ​​the accuracy one can expect from these data.

The data in red have a gap > 10 mn or > 0,10 m with reference SHOM

Should therefore remain cautious and to grant only trusted on these tides calculations. This said, applying a reasonable "pilot's foot" in all circumstances, these predictions can be regarded as generally satisfactory.

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(¹) http://www.fsf.org/
(²) National Ocean Service. A department of NOAA. Data outside the U.S.. are provided for non-commercial use only.