Frequently asked questions

This section gives the answers to frequently asked questions :

What is a maregraph (tide gauge)?

Benchmarks and marigraph references?

What is the purpose of a sensor?

When I ckick on "New Caledonia Wallis and Futuna" on the map on the Home page, I only see the New Caledonia station. Is this normal?

Comment récupérer des données mises à disposition sur REFMAR ?

Quelle est la différence entre des données "temps-réel" et des données brutes à 10 minutes ?

Comment sont calculées les prédictions superposées aux courbes de hauteurs d'eau sur les pages des observatoires ?

Comment sont déterminées les surcotes - décotes ?

Quels sont les facteurs à l'origine des surcotes - décotes ?

Pourquoi n'est-il pas possible de visualiser la totalité des prédictions et les surcotes - décotes ?

Comment puis-je connaître mes droits de téléchargement aux données une fois que je suis inscrit sur REFMAR ?

 

 

► What is a marigraph (tide gauge)?

The term "marigraph" (a.k.a. tide gauge) was introduced by the French hydrographer R. Chazallon (1802-1872), who developed the first apparatus in France, in 1843. Its name comes from the Latin 'mare' (sea) and the Greek 'graphein' (to draw or to write). It is not an instrument that measures the tide but rather the sea level, of which the tide is only one of the phenomena that changes this level.

 

 

► Benchmarks and marigraph references?

The heights of water are cross-referred to a reference plane which is arbitrary and local. These attributes should always be remembered when analyzing tide gauge measurements.

In France, the hydrographic datum or chart datum is a choice that is often found, particularly if the tide gauge has been installed for hydrographic purposes. It should be remembered that the hydrographic datum is defined with respect to a fundamental tidal benchmark and has been chosen close to the level of the lowest astronomical tide. It is always a conventional choice. Readers interested by this question may like to refer to the article published in 
number 79 of the XYZ review by Wöppelmann et al. (1999).

Another choice, encountered more rarely these days, is *a mean sea level calculated over a given period*. For example, the historical tide gauge datum for Marseille is the mean sea level observed between 1885 and 1896. This datum is also known as the French General Levelling datum or, nowadays, as datum NGF-IGN69.

In order to fully alert users concerning this conventional aspect of tide gauge datums, the 
Permanent Service for Mean Sea Level (PSMSL) voluntarily corrects tide gauge datums and redefines them based on a tidal benchmark by an arbitrary figure based on this reference, of around 7000 mm, not linked to any particular sea level (mean or lowest tide) and calls this reference 'RLR' (Revised Local Reference). In this way they stress the relative nature of tide gauge measurement: a local measurement of the sea level with respect to the base on which the tide gauge rests, a base that is represented by a physical reference (a tidal benchmark).

Référence :

Wöppelmann G., S. Allain, P. Bahurel, S. Lannuzel et B. Simon (2011). Zéro hydrographique : vers une détermination globale. Annales hydrographiques 2011, 6ème série, volume 8, n° 777, ISSN : 0373-3629.

 

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