Marigram and marigraph
A marigram is a record of the sea level; since sea level is not constant, the marigram is generally fluctuating. Not only tides but also seiches, forced waves, and long-term fluctuations (stagional or secular) are recorded. Old marigrams were paper strips, with an inked curve; modern marigrams are frequently magnetic or perforated tapes containing a digital record. Paper-strip marigrams could be directly read; taped marigrams must be used with a computer.
A marigraph is a sea-level measuring instrument. The simplest marigraph is a graduated rod, involving a reading by sight only. More sophisticated instruments are recorders, based on various principles (Bruns, 1958).
There are floating marigraphs, pressurometers, ultrasonic (bathymetric ultrasonic survey in reverse, from the stable bottom to the variable surface). At great depths, differential pressurometers are used, in which the constant pressure disappears and only the fluctuation remains. At the present time, surveys by satellite...