3464 HYDROGRAPHIC MANUAL Page 266 



with the depth. It is assumed that a bight is formed in the leadline by the current 

 acting on the submerged portion of the line and the lead, so that when the line appar- 

 ently leads vertically down from the leadsman's hand the lead is actually slightly down- 

 stream and the bight of the submerged portion of the line is still farther downstream. 

 The force of the current puts a strain on the bight so that the leadsman is deceived into 

 believing he is getting a vertical cast. 



To eliminate such discrepancies, therefore, handlead sounding should be done only 

 at slack water or from a boat drifting or running slowly with the current. Crosslines 

 or lines nearly normal to the channel should be run only at slack water and lines in 

 the direction of the current always run with it. 



A similar type of error can be introduced in handlead sounding in strong currents 

 by the leadsman permitting the lead to remain on the bottom for a short interval 

 before reading the depth. In such cases the leadsman should be instructed to read 

 the depth the instant the lead touches bottom. When searching for least depths in 

 an area where current prevails, the lead must be rapidly lifted off the bottom and 

 dropped until the least depth has been found. The lead must not be allowed to remain 

 on the bottom appreciably because the current will form a bight in the leadline to 

 indicate a depth greater than the actual depth. 



Echo soundings are not affected in the above manner; and, when practicable, this 

 method of sounding should be used in areas where strong currents prevail, both because 

 errors in verticality are eliminated and a faster speed and better control of the survey 

 vessel may be maintained. 



Another error may be introduced when sounding in strong currents that affects 

 the spacing of soundings between positions and thus is applicable whether the sound- 

 ings are handlead, wire, or echo. If a launch used in runnmg sounding lines across a 

 channel where there is a strong current is kept on line by the use of ranges, progress 

 along the line will not be at a uniform rate of speed due to the varying strength of the 

 current at various places in the river or channel. In the strongest current the progress 

 across the current is slowed because the vessel must be headed into it more to counter- 

 act its effect. On the contrary, this force will diminish near the edges of the channel 

 and there will be some places where there is no current at all. Insofar as practicable, 

 fixes should be taken at short enough intervals and at places where the strength of the 

 current changes so that the correct spacing of the intermediate soundings will seldom 

 be in doubt. If this is impracticable, such crosslines should be run at slack water. 



Varying cmTents are found in the vicinity of shoals and rapidly changing depths. 

 Even far offshore these erratic currents make the running of the proposed lines difficult. 

 This is especially true where a sounding line crosses the edge of a shoal area where 

 currents of different velocities or directions are found. Sometimes there is visual evi- 

 dence of such a change in the current, but frequently the evidence is not pronounced 

 or sufficiently noticeable to give warning of its presence. An experienced hydrographer 

 will suspect the presence of such currents from the submarine relief, and an attempt 

 should be made to anticipate their effect on the track of the vessel. It is often prac- 

 ticable to make a change in course which will at least partly counteract the effect of 

 the current before evidence of its effect has been given by the consecutive plotted posi- 

 tions. In localities where strong currents of this nature are apt to be encountered, 

 especially if the spacing of the lines is comparatively close, the greatest difficulty is 

 experienced in running the proposed lines accurately enough to avoid the necessity 

 for splits. • . ■ 



