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Utilization of sea level observations

Understanding sea level changes has long been a preoccupation of Man. The first tide tables date as far back as AD 770 to Hangzhou, in China, during the Tang dynasty. The main motivation that led man to take a close interest in the observation of variations in sea level was in all likelihood navigation. Although the first Chinese tables simply showed high and low tides according to the phases of the Moon, the development of maritime commerce gave rise to a need for more detailed understanding of the variations in sea level. This necessity drove Man to imagine increasingly sophisticated ways of accurately measuring the fluctuations in sea level; first, by simply placing tide poles near ports ; later, by designing highly technical instruments like, for example, pressure-operated tide gauges or acoustic tide gauges; finally, in recent years, by taking advantage of artificial satellites.

Tide gauges are still widely used today for the purposes of navigation in coastal waters. However, the data they produce is increasingly used in numerous other applications, the aim of which is the study of our environment and the physical mechanisms which control it, and coastal development. The following are some of the possible uses of sea level data obtained with tide gauges :

- the study of the secular changes in the mean sea level. Today it is believed that the overall mean sea level is rising but we are unable to accurately establish the size of the eustatic signal, nor whether this rise is accelerating.

- understanding the physical processes which result in variations in the mean sea level in any given place (tectonic, artificial subsidence , hydrodynamic, etc.);

- validation of the climate models by comparing their results with the observed sea level trends;

- the calibration of satellite radar altimeters (Topex/Poseidon, Envisat, Jason etc.);

- the study of the transfer function between the observed signal off the coast by spatial altimetry and that measured on a coast with a tide gauge;

- the evaluation and verification of the spatial altimetry results. The drift introduced by an algorithm error, detected in 1996, showed the importance of constantly comparing the results obtained using independent measuring techniques;

- unifying the levelling networks. The reference surfaces of the national altitude systems are only consistent to the nearest metre. This observation can be explained by the origin of each of them, defined at the mean sea level on one coastal site;

- an improvement in the tide models, particularly in complex coastal zones (bathymetry, coastline, non-linear waves etc.);

- the influence of tides on coastal ecosystems : fauna, marine cultures etc.

- the study of extreme events and their consequences: storms, tsunamis, marine flooding...

- the study of vertical references and their mathematical relations (geoid ; ellipsoid ; hydrographic datum; mean sea level etc.) ;

- the reduction of bathymetric soundings. The depths measured are defined relative to a reference level called the "hydrographic datum" or nautical chart datum. The definition of this datum is related to the fundamental reference used to measure water levels. Following a recent recommendation from the International Hydrographic Organisation (IHO), this fundamental reference should in future be a world reference ellipsoid.