5522 



HYDROGRAPHIC MANUAL 



Page 512 



For echo-sounding instruments, especially those with outboard acoustic units, 

 used in launches and small vessels, an alternate method of determining the instrumental 

 error is by means of the bar check which is described in 557 and 5571. When good 

 results can be obtained by bar check, it is the more accurate method and should be 

 used. 



5522. Record of Simultaneous Comparisons 



Simultaneous comparisons should be recorded in the Sounding Record at the time 

 the.y are made. Rubber Stamp No. 35 (fig. 178) should be used for this purpose, the 

 uncorrected echo soundings and vertical soundings being entered in columns under the 

 appropriate headings. The computation of the instrumental correction should not be 

 made in the Sounding Record. 



Simultaneous comparisons should be tabulated in vertical columns and corrected 

 in standard form, according to the example below. The echo depths are corrected for 

 velocity of sound and settlement and squat; and the vertical measurements for any 

 errors of graduation or calibration. The instrumental corrections, obtained by sub- 

 tracting the corrected echo depths from the corresponding corrected vertical depths, 

 should be entered in a separate column. The mean instrumental correction for each 

 set of observations is entered in the "Remarks" column. 



The simultaneous comparisons in a project area for a season should be tabulated 

 chronologically and be included with the computations of velocity corrections (see 8332). 



For simultaneous comparisons, when a graphic-recording echo-sounding instru- 

 ment is used, the depths by bar check, leadline, or wire, should be recorded on the fatlio- 

 gram as well as in the Sounding Record. A mark should be made on the fathogram 

 to identify the exact position of the echo-sounding depth and the corresponding 

 measured depth should be recorded closely adjacent to the mark. 



5523. Frequency of Determination 



The frequency with which the instrumental error should be determined depends 

 on the operational constancy and stability of xhe echo-sounding instrument. If the 

 instrument appears to be in perfect working condition and there is no reason to suspect 

 that the instrumental error has changed, a determination made once each week should 

 suffice. But if the instrument is operating poorly and there is reason to believe that 

 the instrumental error varies considerably, it should be determined at least once each 



