Are the tidal predictions reliable ?

All navigation software offer predictions of tides, more or less long range, that cover the coasts of the whole world. 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 seafront have an organization responsible for hydrography, oceanography and sometimes maritime cartography. In France, the Service Hydrographique et Océanographique de la Marine performs these tasks. The establishment of tidal predictions represents costs assumed by taxpayers, and therefore, results are rarely made freely available to the public. In France, tidal predictions are provided with annual books (SHOM Annuaire des marées, Breton Sailor's Almanac) sold to the public or digital files sold to interested organizations (software and nautical guides editors, local administration, aso.).

This is the reason why the majority of navigation software publishers use free program predictions. XTide.

XTide is a program developed since 1998 couple David Flater, a member of Free Software Foundation (¹). It uses harmonic analysis and tide prediction calculations provided by the NOS (²) des U.S.A. as well as the harmonic files thereof. These data are provided for a large number of so-called “reference” stations and the calculation makes it possible to extrapolate the differences for other stations., called “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

The harmonic data provided by XTide absolutely do not take into account local particularities which may influence the tides actually observed. This is why it is provided without any guarantee and disclaiming all responsibility for the use that may be made of it..

Geographic configuration can significantly change the predictions calculated for a station, and only long-term observation, by appropriate measuring means, allows you to correct calculations to obtain the best precision. This is particularly the case for estuaries., or very closed bays. National services such as the SHOM perform locally such measures, wherever the calculations prove insufficient.

Despite this, 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 the case of Hahn Software which has spent a lot of time in recent months correcting its application data AyeTides and AyeTides XL.

Comparison

A quick comparison table allows you to measure the differences between the different predictions provided by the applications. This is just a “photograph” of a few stations over a single day., with a given tidal range (coefficient 85) and the variations may be more or less important depending on the range. However, this allows us to get an idea of ​​the precision that can be expected from this data..

Data in red has a gap > 10 mn or > 0,10 m with the SHOM reference

Should therefore remain cautious and to grant only trusted on these tides calculations. This said, by applying a reasonable “pilot’s foot” in all circumstances, these predictions can be considered generally satisfactory.

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