Positions at Sea
The geographical location at which an observation is made must be known. The greater frequency with which observations are now taken makes it necessary to know the locations of sampling very accurately, and hence special methods of determining positions at sea have been developed. Accurate knowledge of locations is particularly necessary in surveying, where the introduction of sonic sounding methods has made it possible to take large numbers of soundings that must be precisely plotted in order to bring out the true configuration of the bottom. The specialized techniques developed by such organizations as the U. S. Coast and Geodetic Survey have not yet been used for general oceanographic work, but the methods may be adopted for the study of special problems.
When in sight of land where recognizable features are accurately located, the position of the vessel may be determined by means of horizontal angles and bearings on shore features. Out of sight of land the position can be determined by astronomic sights or by radio direction-finder bearings. Between positions established in these ways the location at any time is obtained by dead reckoning—namely, from the course steered and the distance run. Such methods are adequate for most oceanographic work, but, where greater accuracy is required, as in offshore surveying, positions found in this way are not commensurate with the accuracy and frequency of soundings. In certain cases an anchored vessel or buoy whose position can be exactly established by repeated astronomic observations is used as a point of reference. Since 1923 the U. S. Coast and Geodetic Survey has experimented with sonic methods of locating positions and has developed them to a high degree of accuracy. In radioacoustic ranging (usually designated as R.A.R.), the surveying vessel drops a depth bomb that is fired by fuse or electricity. The sound of the explosion is picked up by a hydrophone on the vessel and recorded on a chronograph. The impulse of the explosion, which travels in all directions, is picked up by hydrophones attached to shore stations, anchored vessels, or buoys whose positions are accurately known. The hydrophones are connected to radio transmitters, and the sound impulse received at each hydrophone is transmitted by radio to the surveying vessel, where the times of reception are automatically recorded on the chronograph. Since the time required for the transmission and reception of the radio signal is infinitesimal, the period between the bomb explosion and the return of the signal from each hydrophone is that required for the sound impulse to travel through the water from the point of the bomb explosion to the hydrophone. The velocity of sound in water can be computed from the known distribution of temperature and salinity
Taut-wire traverses are also used to determine accurately the distances between anchored buoys when detailed surveys are made out of sight of land. The distances are measured by paying out steel piano wire under controlled tension over an accurate meter wheel from drums which carry over 140 miles of wire. When the distance between a row of anchored buoys 15 or 20 miles long has been measured, the wire is cut and abandoned. With a combination of the methods outlined above, extremely accurate surveys can be made out of sight of land where depths are not too great to make it impossible to anchor the buoys satisfactorily. The methods employed by the Coast and Geodetic Survey have been described by Rudé (1938) and by Veatch and Smith (1939).