3375 HYDROGRAPHIC MANUAL PaGE 236 



3375. Wind and Current 



Dead reckoning can be run most accurately in a dead calm, but this condition 

 cannot always be waited for. Experiments should be made to determine the amount 

 of leeway which will be caused by winds of different velocities at various angles to the 

 survey ship's heading. 



An allowance must be made for current and, lacking observational data, an estimate 

 may be based on the following general information: 



(a) Observations have demonstrated that a persistent wind will set up a wind-driven current with 

 a velocity approximately 2 percent of that of the wind. This ratio can be expected to hold both in 

 coastal areas and in the open ocean. 



(b) The direction of a wind-driven current in the Northern Hemisphere has been found to be 

 generally about 20° to the right of the wind in coastal areas, but theoretically this deflection is prob- 

 ably nearer 40° to the right of the wind in the open ocean. The rule for coastal wind-driven currents 

 is not always applicable, especially when near the shore where the direction of the current depends 

 on the angle between the wind direction and the coastline. 



3376. A Dead-Reckoning Line 



In running a dead-reckoning line, certain precautions must be observed and certain 

 data must be obtained. These involve (a) the initial position, (6) the terminal position, 

 and (c) intermediate data. 



a. The initial 'position. — An initial position for a dead-reckoning line should be 

 selected which is correct beyond doubt. If data complete enough to fix positions are 

 obtained farther offshore, but about whose reliability there is some doubt, they should 

 be used merely as intermediate data to assist in the final adjustment of the dead- 

 reckoning loop, but the line resulting from the first adjustment of the dead reckoning 

 should not be moved far from its natural position in order to utilize such data. 



When practicable, each dead-reckoning line should start from a series of fixed 

 positions, a constant course and speed being maintained during the several positions, in 

 order that these positions may establish the course made good and a log factor for the 

 initial part of the line. 



h. The terminal position.- — As the dead-reckoning line approaches its end, a position 

 must be fixed by conventional means as soon as practicable. If the offshore control is 

 by R.A.R., an attempt should be made to get bomb returns even before these may be 

 expected to give results. A single bomb return will provide a distance arc which may 

 be used in adjusting the dead reckoning. 



When the terminal position is to be a visual fix, especially if observed to buoys, 

 it is essential that the most experienced officer available be on the lookout for the 

 signals to come into view as the survey ship nears the control. The observation of a 

 visual fix while the signals are still extremely distant or indistinct and when their 

 whereabouts is unknown, is one of the most difficult phases of hydrography. Usually 

 an experienced officer can locate two of the objects and set a sextant on the angle be- 

 tween them, passing the sextant to another observer who can maintain that angle, 

 while the more experienced one finds the other object. The end of the line, like the 

 beginning, should be fixed by a series of reliable fixed positions, and they and the more 

 doubtful data should be utilized in the adjustment as described for the mitial position. 



c. Intermediate data. — Between the initial and the terminal positions all control 

 data such as single bomb distances, angles, and bearings must be observed and re- 

 corded, in addition to the dead-reckoning data, for use in the final adjustment of the 

 loop. When the adjustment is made, some of these data will be found to be either in 



