GRGS > Research > Projects > BATHYELLI

Bureau des longitudes Centre national d’études spatiales Institut national de l'information géographique et forestière - Laboratoire LAREG Observatoire de Paris Institut national des sciences de l'Univers Observatoire de la Côte d'Azur Service Hydrographique et Océanographique de la Marine Observatoire Midi-Pyrénées Université de la Polynésie Française Ecole Supérieure des Géomètres et Topographes - Conservatoire national des arts et métiers Ecole et Observatoire des Sciences de la Terre


BATHYELLI (BATHYmetry referenced to the ELLIpsoïd): This project, led by SHOM, aims to measure the vertical hydrographical references for the entire French coastline using a stable, accurate and accessible reference frame (GRS80 ellipsoid of RGF93).

This project, begun in 2005, has the following goals :

  • to be able to change from a vertical reference to another (average level, lowest sea level, chart datum, IGN69, geoid, ellipsoid GRS80 of the RGF93).
  • to be able to reduce the surveys (to change from water height, as measured by the sounder, to depth in relation to the chart datum on marine maps) without having to correct the tidal heights and the meteorological effects, thanks to the use of GPS and the knowledge of the ellipsoidal height from the chart datum.

The calculation of the chart datum is done through the average level. The average level is determined:

  • off the coast, using satellite altimetry, it is the average hydrographic surface;
  • on the coast, using tide gauges; it is derived from analysis of data from tide gauges

To fill the gap between the open sea and the coast, a series of GPS survey campaigns was conducted by the SHOM at 20 sites. These campaigns enabled them to determine the average level at each site. The average level of French coastal areas comes from interpolating these three data sources (satellite altimetry, tide gauges and GPS).


All the countries bordering the North Sea also determined the chart datum in the ITRS (International Terrestrial Reference System, the RGF93 is 3the French project of the ITRS). The goal is to have a common surface of reference levels in the North Sea.