OCEANOGRAPHIC INFORMATION FOR SUBMARINE CABLES 1051 



that it be compiled, evaluated, and culled with due regard for the sources, 

 inherent errors, and possible misinterpretations. The data remaining 

 after this type of review can then be combined with other knowledge 

 from the earth sciences to provide the best available maps and profiles. 



2.2 Instrumentation for Topographic Studies 



2.2.1 Navigation 



Celestial navigation, depending on hand sextant sights of heavenly 

 bodies and subsequent solution of an astronomical triangle, is the only 

 method available in all parts of the world. It is, of course, useless during 

 periods of poor visibility. Positional accuracy of ±| mile can be obtained, 

 although the usual accuracy is ±2 to 5 miles. During the interval 

 between sights, an estimated position is computed by dead-reckoning 

 methods, assuming the ship's speed and course to be known. Speed 

 determined by propeller revolutions or by a pitometer log is subject 

 to the influence of winds, seas, and currents. The ship's heading is 

 indicated by compass, but the course actually traveled by the vessel 

 is affected by all the same variables that affect the speed. Starting 

 from a celestial or radio fix, a good navigator can plot a dead-reckoning 

 track with little error over short periods of time under ideal conditions. 

 The probable error increases rapidly with elapsed time under less than 

 ideal conditions. 



Some of the existing radio navigational systems and their limitations 

 are listed in Table I. 



2.2.2 Echo Sounding Errors and Corrections 



Present-day echo sounders, operating with a total beam angle of 

 about 60 degrees, indicate the chstance to the best reflectors within 

 the effective cone of sound. Neglecting scattering layers, which can 

 usually be eliminated from consideration by interpretation, the best 

 reflector will coincide with the bottom immediately below the transducer 

 if the bottom is horizontal or if the highest point on the bottom is 

 immediately below the transducer. Under any other condition the echo 

 sounder indicates less than the true depth. Where the bottom is very 

 une\'en or rocky, a multiplicity of echoes are returned and recorded. 

 These considerations necessitate careful evaluation of the data and 

 its correction for slope. 



Since the echo soimder actually measures the time interval between 

 transmission of a pulse and receipt of an echo, the timing mechanism 



