57 HYDROGRAPHIC MANUAL ' Page 542 



Another consideration to bear in mind is that echo-sounding instruments are being 

 used increasingly for navigation, and if the navigator uses a sonic instrument he also 

 will read uncorrected depths. If from a sm'vey vessel at a given point an echo sounding 

 is obtained with an eiTor due to bottom slope, and this uncorrected depth is charted, 

 it will represent the depth which the navigator will obtain on his instrument at the same 

 point. If such soundings should be corrected for errors due to slope, it would be neces- 

 sary for the navigator to reverse the correction process or to apply corrections to the 

 echo soundings he obtains before he could utilize the charts. 



Because of the complexity of the problem and the difficulty of applying slope cor- 

 rections rationally, especially in irregular bottom, slope corrections should not be made 

 by field parties except as specifically directed in the project instructions. 



In areas of irregular bottom and steep slopes, the reflected echo is often registered 

 on the echo-sounding instrument indistinctly, or as a multitude of depths scattered 

 along the scale, one of which may be the true depth. On a visual instrument the selec- 

 tion of this true depth is virtually impossible, due to its confused character and the 

 fact that the observer has no time for analysis. On the fathogram of a graphic-recording 

 instrument, however, the characteristics of such soundings appear more clearly, and 

 there is a permanent record of them from which a deliberate study may succeed in 

 selecting the true vertical depth, or one near it. At any rate the evidence of what was 

 registered on the instrument is permanently recorded so that the results can be more 

 properly evaluated than is possible from a series of depths recorded in a Sounding 

 Record. 



In connection with bottom slopes, caution is necessary in examining fathograms 

 of graphic-recording instruments. Because of the exaggerated vertical scale one is 

 apt to be startled by the steep slopes depicted and to think of them as representative 

 of the actual submarine relief. This is far from true since it is customary on most 

 fathograms to have the vertical scale increased many times with reference to the 

 horizontal scale. For the fathogi-am of an 808 Fathometer the vertical scale along the 

 arc is about 45 times the horizontal scale for a record made from a vessel traveling at 

 7 knots, and about 65 times for a vessel traveling at 10 knots. From these relationships 

 the relation of the vertical scale to the horizontal for other vessel speeds can be com- 

 puted. 



57. ECHO-SOUNDING INSTRUMENT OPERATION 



Although the fathometer attendant is not expected to be able to repair and make 

 mechanical adjustments to an echo-sounding instrument, he should be familiar with 

 the principles of operation of the instrument he is using; and he should follow certain 

 precautions in operating the instrument and in reading the registered depths in order 

 that the recorded values may most accurately represent the true depths. 



The various precautions to be observed in operation and in reading the registered 

 depths are described in chapter 3 and in this chapter, but they are summarized here 

 for convenience, and unless otherwise noted apply to all echo-sounding instruments 

 described in this Manual. 



(a) The gain control must be adjusted at the highest vaUie that will not produce excessive 

 strays or cause oscillation of the amplifier, and it should be maintained at this value, except when it 

 is necessary to change it for the proper registration of soundings. This setting should be the same as 

 was used in any determination of the instrumental error or in any comparative soundings (see 552). 

 It must be borne in mind that a readjustment of the gain control alters the position of registration on 

 the depth scale. (See 5163.) 



