BATHYELLI project (BATHYmetry relative to the ELLIpsoid)


Recommendations on vertical datums


The International Hydrographic Organization (IHO) recommends in its publication S-44 (standard for bathymetric surveys) that "Ideally [this] sounding datum should also be a well defined vertical datum such as, LAT, MSL, a geocentric reference frame based on ITRS or a geodetic reference level." 

SHOM has chosen to reference its vertical datums relative to the GRS80 ellipsoid of the IAG (International Association of Geodesy). The geodetic reference frame used is the official system RGF93 which is a French geodetic network (established by IGN) of the ITRS.


Vertical datums


The BATHYELLI project, which stands for "bathymetry relative to the ellipsoid", references the following vertical datums to GRS80:

  • Hydrographic mean sea surface (HMSS): this is obtained by combining the mean sea surface (MSS) derived from satellite altimetry and a coastal MSS based on GPS surveys specific to SHOM;
  • Equilibrium level: this is a mean sea level determined from the HMSS refined by the longest tidal observations available to SHOM; 
  • Lowest astronomical tide (LAT);
  • Chart datum (used for nautical charts and sounding reductions);
  • The vertical datum IGN69,
  • The European quasigeoid model EGG97.


With the RAM product (Maritime Altimetric References), users now have individual information for each reference port in Metropolitan France. With BATHYELLI, a surface model of vertical datums will be available.


variation zero hydrographique




The BATHYELLI project began in 2005. The first stage of the project consisted of defining the methodology of GPS surveys that complement the mean sea level data:

  • offshore: satellite altimetry;
  • in ports of reference: tide gauges;
  • between the two: GPS surveys conducted by SHOM in 2006-2008. 


The first vertical datums were generated in late 2009 for the English Channel-Atlantic and the Mediterranean. The "theoretical" accuracy aimed at for the chart datum was within approximately 10 centimetres.

A second phase of the project was initiated in mid-2011:

  • the goal is the validation and improvement of vertical datums, by including additional GPS measurements;
  • this step is also a "production start" for SHOM: new methods of acquisition and processing to perform hydrographic surveys directly at the ellipsoid will be defined;
  • a critical capability will be developed: different software tools will make it possible to change from one vertical datum to another.


To find out more:



  • Pineau-Guillou L. (2009) Projet BATHYELLI : Détermination du zéro hydrographique à partir de l'altimétrie spatiale et du GPS. Revue Navigation Avril 2009, volume 57, n°226
  • Pineau-Guillou L. and L. Dorst (2011). Creation of vertical reference surfaces at sea using altimetry and GPS. Annales hydrographiques 2011, 6ème série, volume 8, n°777, ISSN : 0373-3629.



Last updated: 12/12/2012

List of projects based on tide gauging:

List of programs, projects and reconstructions of tide gauge data