Height references

Observing the sea level and using these measurements to determine several height references is one of the basic hydrographic applications of tide gauging to reduce soundings to the same vertical datum, the chart datum.

To measure the water height, a reference with two fundamental qualities must be established.

  • It must be clearly recognized, which means that when a height is given, we know where it was measured from;
  • It must be permanent, which means that it must be defined precisely or be positioned relative to permanent benchmarks.

Mire positionnée sur un repère de nivellement - Observatoire marégraphique de l'Ile d'Aix (Crédits SHOM, Nicolas Pouvreau, mars 2010) Cliquez sur l'image pour l'agrandir

 

Problems in selecting and locating the vertical datum

 

The choice and location of vertical datums are fundamental problems in hydrography.

These problems are long-standing, but new technologies such as satellite observations and new computer technologies are profoundly changing the way they are addressed.

What with positioning systems such as GPS / GLONASS / Galileo, altimetry, the unique international geodetic reference ITRF, the improved determination of the Earth's gravitational field and ever more powerful hydrodynamic models, new systems of reference are being considered.

However, traditional systems are still in use, and it is necessary to precisely relate these reference frames to one another.

 

Schéma récapitulatif des niveaux de marée (cas des marées semi-diurnes). Figure extraite du produits RAM 2011, SHOM - Cliquer sur la figure pour l'agrandir.

 

Some typical sea levels in France

In France, some typical sea levels are used as a reference for height measurements, including:

 

Mean sea level at Marseille

"The mean sea level at Marseille" at the end of the nineteenth century defined the reference for a zero officialised on January 13, 1860 called "zero of Bourdaloue" or the zero of France's fundamental levelling benchmark.

It was transposed by geometric levelling (spirit level) for the general levelling of France. From 1857 to 1970, three levelling networks succeeded one another: Bourdaloue networks, Lallemand networks and IGN 69 networks.

 

Repère fondamental scellé dans la cave de l'observatoire marégraphique à Marseille (Crédits LIENSs, Nicolas Pouvreau, juin 2004) Cliquer sur la figure pour l'agrandir.

 

The altitude of the fundamental benchmark was set based on the average tidal observations made in Marseille between 1885 and 1897. The fundamental benchmark is a platinum rivet sealed in the Anse Calvo tide gauge building.

 

Lowest astronomical tide

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