Terrestrial benchmarks and levelling

In ports, the chart datum and the nominal mean sea level are measured relative to benchmarks. These benchmarks are located near the tide gauge and are sufficient in number and adequately spaced so they cannot be destroyed simultaneously, for example during port construction work.


Exemple de schéma de situation en élévation des repères d'altitude et des différents zéros de référence pour l'observatoire marégraphique d'Ajaccio-Aspretto (extrait de la FOM SHOM, juin 2010) Cliquez sur l'image pour l'agrandir



They are referenced relative to one another by geometrical levelling, and if possible referenced to general levelling.



Nivellement des repères de nivellement de l'observatoire marégraphique de Socoa - Saint-Jean de Luz (Crédits SHOM, GHA, juillet 2004) Cliquez sur l'image pour l'agrandir Nivellement_maregraphe Tubuai GHO 2008.JPG avec en infobulle : "Nivellement du marégraphe radar de l'observatoire de Tubuai (Crédits SHOM, GHO, 2008) Cliquez sur l'image pour l'agrandir


It should be noted that the chart datum is not defined by its measurement in the general levelling. It is important for chart datum not to be called into question by successive levelling operations which may give different results, not only because of changes in measurement technology, but also due to potential vertical land movement caused by seismic or tectonic events. Even ocean tide loading can cause vertical movement (periodic like the tide) of the benchmarks (referenced to the geoid) exceeding 20 cm; these movements are not significant locally for hydrography since it is a movement of the entire base that observations and models naturally take into account to achieve centimetre accuracy when using satellite technology.


To find out more:



  • Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.


Last updated: 12/12/2012