Page 539 echo sounding 562 



It should be emphasized that this method is not to be used except when exception- 

 ally good results are obtained from the bar checks and they are consistent enough to 

 give reasonable assurance that the greatest part of the difference is due to the velocity 

 of sound. Furthermore, the method is applicable only where the range of the bar depths 

 nearly covers the range of depths sounded. Velocity corrections must not be extra- 

 polated from a correction curve so obtained, for depths greater than one-third more than 

 the range of the bar checks. 



562. Mechanical Corrections to Fathogram Soundings 



An instrument has been designed in the Coast and Geodetic Survey by which 

 depths can be scaled from the fathogi-am of an 808 Fathometer fully corrected for tide, 

 phase, and index setting, and approximately for the velocity of sound. The instrument 

 consists of a small bench with a reel at each end to hold the fathogram record spools, 

 between, which is a platen across which the fathogram may be moved, assimilating 

 its movement at the time the record was made on the echo-sounding instrument. As 

 the fathogram is moved across the platen, it passes imder a circular piece of transparent 

 plastic about 10 inches in diameter, on which is inscribed a circidar graduated scale 

 covering the entire range of depths (0 to 160) of the 808 Fathometer. The scale is 

 graduated for the calibration velocity of 820 fathoms (or approximately 1500 meters) 

 per second, but the circular plastic scale is removable, and others are provided with 

 scales graduated for velocities of sound at 15-meter intervals, i. e., for velocities of 

 1470, 1485, 1515, etc., meters per second. 



The radius of the circular scale is exactly equal to the length of the stylus arm of 

 the Fathometer, so that the circumferential arc of the scale corresponds to the arc 

 described by the stylus across the fathogram. Provision is made for keeping the fatho- 

 gi'am alined horizontally w4th the center of the scale. For scaling any given recorded 

 depth, the center of the circular scale will occupy the same relative position as was 

 occupied by the center of the stylus arm at the time the record was made. 



On the periphery of the plastic scale are four gi'oups of graduations, each ranging 

 from -f 5 to —20, to provide for tide corrections for the different phases of the instru- 

 ment (see 5232). 



In scaling depths with this instrument the printed scale on the fathogram is ignored. 

 The following four adjustments ai'e provided: (1) The circular plastic scale can be moved 

 vertically, with reference to the fathogram, to make the circular scale coincide with 

 the arc drawn by the stylus arm ; (2) the scale can be rotated to set the zero of the scale 

 correctly with reference to the recorded position of the transmitted signal on the 

 fathogram; (3) it can be rotated also to correct for the height of tide; and (4), it can 

 be rotated to scale depths from any one of the four depth phases which the instrument 

 provides, without disturbing the above three adjustments. 



The 808 Fathometer operates at a calibration velocity of 820 fathoms per second, 

 and a scale graduated for this calibration velocity is provided. But by changing scales 

 and always using the scale for the velocity of sound nearest to the average which 

 existed from surface to bottom in the depth sounded, no error exceeding one-half percent 

 of the depth will be introduced by this method of correcting for velocity. 



Or if accurate bar checks have been obtained tlu-oughout the range of depths 

 sounded (see 5617), a scale can be selected to fit the record of the bar check without 

 any reference whatsoever to the velocity of sound. 



