Until quite recently the use of float gauges was the most common method of measuring and recording sea levels.
The float gauges were installed in stilling wells that dampened the wave action and, through a system of wires and pulleys, the vertical movement of the float was translated to a pen that traced out a line on a paper chart attached to a rotating drum. Towards the end of the 20th century the pen and paper chart recording system became superseded by digital encoders that allowed sea level readings to be stored electronically, or transmitted by telephone or satellite.
Today there are a number of different types of technology to choose from to make sea level measurements. Some systems are mounted above the water and determine the distance down to the water surface by measuring the time taken for a pulse of sound or radar to travel down to the water and be reflected back to the source. Other systems are placed under the water and measure the water pressure which is converted to the height of the water column above the sensor.
At a particular port the level of the water is expressed as a height above a local datum which is also the datum used for the depths of the sea on nautical charts. This datum is defined with reference to permanent benchmarks ashore and the zero of the tide gauge. The datum adopted usually approximates Lowest Astronomical Tide (LAT) which is the lowest level the tide can be predicted to occur under normal meteorological conditions.