SHOM BATHYELLI: new datum product referenced to the ellipsoid on

Version 1.0 of SHOM's BATHYELLI Product "Chart Datum referenced to the ellipsoid GRS 80 of RGF93" has been available for download on the site (with specifications in French and English) since 10 December 2013. No subscription is required to retrieve these public data under an Open data license. The product was developed and validated based primarily on tidal observations.

Capture d'écran - surface BATHYELLI Zéro Hydrographique / ellipsoïde (CD/GRS80)


BATHYELLI project (BATHYmetry relative to the ELLIpsoid)

The height of the chart datum is defined by SHOM in the legal altimetric systems of reference (Decree n ° 2006-272 of 3 March 2006). This chart datum as well as several characteristic sea levels useful in hydrography were previously published periodically for each tidal zone (SHOM Maritime Altimetric Reference Values). The BATHYELLI project aims to generate this information in the form of continuous vertical reference surfaces. This opens up many applications, including change of reference for land-sea continuity, hydrodynamic modeling and bathymetric surveys directly referenced to the ellipsoid, which are now possible along the coast as well as at sea.


A crucial application for hydrography: bathymetric surveys referenced to the ellipsoid

This so-called "compensation" surface makes it possible to conduct bathymetric surveys relative to the ellipsoid, an important technical leap in hydrography.

Levé classique vs levé bathymétrique rapporté à l'ellipsoïde (Crédits SHOM)

Standard survey vs. bathymetric survey relative to the ellipsoid (Photo credits SHOM)

During a survey, vessels measure the sea height. It is necessary to correct this raw measurement for the tides and any variations due to weather (surges) to determine the depth relative to the datum: it is often necessary to set up a tide gauge for the duration of the survey.

But when using vessels equipped with GPS/GNSS receivers and a BATHYELLI model of the chart datum relative to the ellipsoid, it is no longer necessary to correct for the tide and weather to obtain the depth measurement!


Limit for obtaining reliable ellipsoid heights

Previously the difficulty lay in determining the tide, but even in surveys relative to the ellipsoid, difficulties still persist.

Main limitations

Obtaining reliable ellipsoid heights is critical:

  • geodetic techniques using kinematic post-processing may be necessary,
  • hybridization with the data from the ship's attitude and heading reference units can also improve accuracy.

First results comparing a traditional survey and a survey using a GNSS receiver

SHOM conducted three survey experiments in the second half of 2013 and the preliminary results are encouraging. In water over fifty meters deep, the difference between the bathymetric data obtained by conventional reduction and bathymetric data relative to the ellipsoid is between:

  • 10 cm offshore of Le Conquet,
  • up to 30 cm offshore of Le Contentin.


Frequency of updates

SHOM's BATHYELLI product will be updated annually to include changes and improvements.


Review of vertical datums at sea

The chart datum is the reference plane for nautical charts and tide tables. This reference at sea differs from the reference elevations on land maps (IGN69). All bathymetric data (depths) are given relative to chart datum. Soundings are counted positively downwards from the datum.

Références verticales en mer (Crédits SHOM)

Vertical datums at sea (Photo credits SHOM)

The chart datum is defined near the level of the lowest astronomical tide to ensure that there is at least as much water as what is indicated on the nautical chart.

Two other important vertical datums in hydrography are:

  • the mean level, used for modeling currents and waves,
  • the highest astronomical tide, which defines the coastline.

Thanks to advances in satellite technology, a new vertical datum known as the ellipsoid has been developed, which is a mathematical model of the planet used to express the height data obtained when using a GPS receiver. This geometric reference has the advantage of being stable (not related to sea levels, which may change over the long term), accurate and accessible anywhere at sea.

IHO recommends referencing to the GRS80 ellipsoid associated with the ITRS, which is the most accurate global geodetic system. The BATHYELLI surfaces include the vertical datums described above, covering metropolitan France (excluding estuaries) and are referenced to the French RGF93 geodetic system of the ITRS.



  • GRS80: Geodetic Reference System 1980, ellipsoid based on the ITRS
  • RGF93: French geodetic reference system 1993




Pour en savoir plus

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