The velocity of sound in water is a function of temperature, salinity and pressure. When an echo-sounder is used to make depth measurements, the time taken for a pulse to reach the ocean bed and return is measured. The machine is set to a standard sounding velocity, usually 800 or 820 fm/second (1463/1500 m/sec), and a nominal depth is recorded. This depth should then be corrected for the properties of the water. The ‘Tables of Velocity of Sound in Pure Water and Sea Water” by Matthews are commonly used for this purpose. Matthews compiled his tables using soundings taken by a number of research ships all over the world. He divided the oceans into 52 areas, and for each area calculated a set of depth corrections from 200 m to the ocean bottom. Since the tables were published more accurate measurements of velocity as a function of temperature, salinity and pressure have been made. There are also many more observations than were available when the tables were compiled. However, as these tables are in general use, no attempt has been made in this paper to change the corrections, and any computer results are therefore no more accurate than the tables.

When correcting depths manually aboard ship, the following procedure is followed. The depth on the echo-sounder is noted and entered into a log book. The position of the ship is known using some standard method of navigation such as satellite, Decca, Loran or radar.