Over the last two decades, while satellite radar altimetry has been revolutionizing our understanding of the oceans, there has also been an unprecedented development of tide gauges, both in terms of the technology used and also their applications. The increased need for coastal observations of sea level, with data in digital form that is easily accessible using modern means of communication, goes beyond the traditional boundaries of hydrography, sea-level prediction or navigation. It is in this context that SONEL, a system designed for the observation of the level of coastal waters, was born. Today it has entered a demonstration phase and relies on voluntary participation.
The main aim of SONEL is to gather and make available measurements of sea level on the coast which have been obtained using tide gauges of the highest metrological quality possible, to provide researchers with data that will enable them to answer the important questions of today concerning the changes that affect the different components of sea-level : is the mean sea-level rising? Where? How? Why? Are storm waves more frequent today? etc. The answer to these questions depends on the acquisition of a long time series of sea-level measurements. The synergy between tide gauges and other measuring techniques is also a key point in their interpretation and particularly with spatial techniques for precise positioning (GPS, DORIS etc.) which can be used to verify the geodetic stability of the land on which the tide gauges are located.