5616 HYDROGRAPHIC MANUAL PaGE 538 



5616. Instrumental Adjustment to Account for Velocity 



All echo-sounding instruments used by the Coast and Geodetic Survey are operated 

 at a constant motor speed for a selected velocity of sound known as the calibration veloc- 

 ity, and subsequent corrections are made for any difference between the actual and the 

 calibration velocity. It is possible to allow for the velocity instrumentally if the aver- 

 age value of the velocity of sound is known in advance.' Assuming that the index is 

 properly set to account for the draft of the acoustic units (see 554), it is possible to regu- 

 late the speed of the registering device, or to change the position of the index on the 

 scale, so that the registered depths need no further correction for velocity. The latter 

 method may be used in limited degree on some of the echo-sounding instruments used 

 by this Bureau (see a below). Instruments have been designed and used which are 

 provided with a simple means of altering the speed of rotation of the driving motor to 

 make the registered depths correct for the assumed velocity conditions (sec 529). 



Adjustments of this nature are of restricted application, A rather detailed knowl- 

 edge of the average velocities of sound in the project area must be available before 

 actual sounding. Furthermore, since the velocity of sound usually varies with depth, 

 the use of such an adjustment is only applicable throughout a limited range, and pre- 

 supposes a knowledge of the depths to be measured. 



a. Compensation by index setting. — For shoal-water sounding, where the range of 

 depths sounded is limited and reliable results are obtained by bar check (see 557) , it is 

 possible to adjust the index to compensate for the error of soundings at a mean depth 

 of the range. This method of compensation may be used where no residual error 

 greater than 0.25 foot in depths less than 10 fathoms or greater than one-half percent in 

 depths greater than 10 fathoms is left at either limit of the range of depths. For 

 example, if the range of depths sounded is from 10 to 60 feet, and the bar check discloses 

 an error of 0.5 foot at 10 feet and 1.0-foot error at 60 feet and other errors are propor- 

 tional or nearly so, the index can be adjusted to compensate for the error of 0.75 foot 

 at 35 feet, leaving residual errors of 0.25 foot of opposite signs at the limiting depths 

 of 10 and 60 feet. Or, if the range of depths sounded is from 10 to 20 fathoms, and the 

 bar check discloses errors of 0.1 and 0.25 fathom at 10 and 20 fathoms respectively, and 

 other errors are proportional or nearly so, the index can be adjusted to compensate for 

 an error of 0.15 fathom at 13}3 fathoms, leaving residual errors of 0.05 and 0.1 fathom 

 of opposite signs at the limiting depths of 10 and 20 fathoms respectively, the error in 

 each case being one-half percent of the depth. 



5617. Velocity Corrections by Bar Check 



For sounding in shoal water, the bar check (see 557) can be used to derive cor- 

 rections applicable to the various depths. With the index of the instrument properly 

 adjusted (see 5545), the results of accurate bar checks may be used to plot correction curves 

 for use in correcting the registered depths. Only bar checks taken under the most favor- 

 able conditions are considered sufficiently accurate for this purpose. When used for 

 this purpose, the mean differences between the bar depths and the registered depths at 

 each depth shall be used as a correction to apply to all soundings of that depth. These 

 differences resultmg from the bar check should be plotted as a correction curve. The 

 hydrographer must use his judgment as to whether the results of a number of bar checks 

 should be averaged to be applied over a considerable period of time, or whether the 

 result of the bar check at the beginning of the period should be averaged with that at 

 the end of the period for application to the soundings taken between bar checks. 



