An instrument used to measure the height of the sea waters during a certain period of time. Data gathered with these instruments, registered graphically on the so-called maregrams, are used to calculate the mean sea level as well as to study tidal behaviour. Their observations are also used to calculate the hydrographic zero (height set somewhat arbitrarily a little below the minimum sea-level) and the level of reference in cartography.

© Instituto Camões 2004-2005