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HYDROGRAPHIC MANUAL 



depth recordings which must be rejected or 

 laboriously corrected. See Chapter 3. 



5-106 Draft correction. — Draft, instru- 

 mental error, and settlement and squat are 

 factors for which compensation may be made 

 by an appropriate adjustment of the instru- 

 ment. The effect of each of these is deter- 

 mined individually and then combined 

 algebraically into a total for which allow- 

 ance is made by setting the index of the 

 instrument. 



The depth of water registered by an echo 

 sounder should be the depth below the sur- 

 face of the water, and not the depth below 

 the submerged acoustic units. Draft with 

 reference to echo-sounding corrections shall 

 be understood to mean the depth of the 

 transducer below the surface of the water 

 when the ship is not underway. 



Provision must be made, or special in- 

 struments must be installed, to measure the 

 draft of the acoustic units permanently 

 mounted in the hull. An internal draft gage 

 may be installed by which the draft of the 

 units may be read directly. The method 

 most commonly used is to mark points on 

 the rail or deck above and abeam of the 

 units. Knowing the vertical distance of these 

 marks above the units, their draft can be 

 determined at any time by measuring the 

 vertical distance of the reference marks 

 above the water surface and taking the dif- 

 ference between the vertical distances. The 

 reference marks should be established on 

 both port and starboard rails and measure- 

 ments to the water surface made from both 

 points. The elevation of the reference marks 

 above the acoustic units is determined while 

 the ship is in drydock with the aid of an 

 engineer's level and a steel tape. 



The variation of the ship's draft and the 

 depth of the water are the two factors which 

 determine the frequency of draft measure- 

 ments. For soundings of 10 fathoms or less, 

 the draft should be known within one-fourth 

 foot and should be measured with sufficient 

 frequency to insure this. For soundings 

 deeper than 10 fathoms, the draft should be 

 known within at least one-half percent of 

 the depth of the water. For sounding in 



deep water where the possible error due to 

 erroneous draft adjustment is a small per- 

 centage of the total depth, echo sounders are 

 usually adjusted for a mean draft. 



5-107 Instrumental error. — Errors caused 

 by instrumental time lags are inherent 

 in all echo-sounding instruments, and some 

 have mechanical lags. Their effect on echo 

 sounding is to increase the registered depth. 

 Such errors differ for each type of 

 instrument and are unlikely to be exactly 

 the same for instruments of the same design. 

 Furthermore, such errors are variable in 

 which changes may be attributed to: (1) 

 variation of tuning and gain of the echo 

 amplifier; (2) variation in strength of the 

 transmitted and echo signals; (3) adjust- 

 ment of the keying circuit; and (4) de- 

 terioration of tubes and parts. There are 

 other minor causes of slight errors. Sound- 

 ing at incorrect frequencies or speeds are 

 considered operational errors rather than 

 instrumental errors. 



Perhaps the most troublesome source of 

 instrumental time lag is that due to a varia- 

 tion in the strength of the echo signal. As 

 its strength decreases, the recorded depth in- 

 creases and is always too large. This may 

 be the result of varying the gain of the am- 

 plifier or a variation in the strength of the 

 echo. The amplifier must always be operated 

 at the highest gain practicable without 

 introducing too many strays, and must not 

 be less than the gain setting used when mak- 

 ing bar checks. The strength of the echo 

 signal is partly a function of the depth, but 

 is also affected by the character of the bot- 

 tom and the characteristics of the water. 



The instrumental errors are usually small 

 and are principally additive, but their total 

 may be of such magnitude as to require cor- 

 rection or instrumental compensation. The 

 amount of the error should be determined 

 periodically and compensated for instrumen- 

 tally. It is especially important that this be 

 done where the survey includes precise sound- 

 ing in shoal water. 



There are two methods for determining 

 the amount of the instrumental error: (1) 

 by comparing echo sounding with simul- 



