12 



HYDROGRAPHIC MANUAL 



by the depth of water and scale of the sur- 

 vey. Soundings should be recorded at regu- 

 lar intervals and in sufficient numbers to 

 provide a realistic representation of the sea 

 bottom and submarine relief (see 5-29). 

 Maximum depths in depressions and mini- 

 mum depths on shoals shall be recorded as 

 they occur and exact time or fraction of 

 sounding interval noted in each case. The 

 recording of^n excessive number of sound- 

 ings shall be avoided, as this results in wasted 

 effort when the records are processed (see 

 6-59). When echo soundings are supple- 

 mented by hand lead, pole, or wire soundings, 

 the record shall clearly indicate the method 

 used. 



When sounding in an area of uneven bot- 

 tom the soundings observed at regular in- 

 tervals shall be recorded on alternate lines 

 of the sounding record. Odd interval sound- 

 ings shall be interspersed as necessary to 

 define the submarine topography. 



1-34 Scanning fathograms. — The devel- 

 opment of echo sounding recorders has 

 provided an infinite increase in the number 

 of recorded soundings. The problems en- 

 countered in verification of smooth sheets 

 have also been multiplied. Many of these 

 difficulties can be traced to improper inter- 

 pretation of fathograms or to poor scanning 

 techniques (see 5-122). Every fathogram 

 must be carefully check scanned. A cursory 

 inspection for peaks and valleys is not suffi- 

 cient. The scanner must be particularly 

 careful in scaling soundings in areas where 

 kelp or grass all but obliterate the bottom 

 trace. Soundings are often recorded in the 

 wrong phase or are in error by 5 or 10 

 units, and strays or side echoes incorrectly 

 recorded as. true soundings. 



Incorrect paper speed is an indication that 

 the recorder was not operating properly, but 

 it is not conclusive evidence that the recorded 

 soundings are in error. Corrections to sound- 

 ings should not be applied unless there is 

 other proof that they are necessary. The 

 check scanning is not complete until all sig- 

 nificant variations from the initial setting 

 have been scaled and entered in the record 

 book. 



The Chief of Party or other responsible 

 officer should supervise and inspect this 

 phase of operations. The necessity for care- 

 ful and accurate scanning of fathograms can- 

 not be overemphasized. 



1-35 Sounding equipment. — The follow- 

 ing listed sounding instruments shall be used 

 in applicable depths : 



(a) The sounding pole or lead line in 

 depths too shoal for echo sounders, to supple- 

 ment and verify echo soundings when sound- 

 ing in areas containing kelp or grass, and 

 to verify least depth on shoals, obstructions, 

 and other dangers to navigation ; 



(b) EDO 255 or 808 type recorders oper- 

 ating on feet or fathoms scale in compara- 

 tively shoal waters, and to the limit of the 

 scale in deeper waters when deep water re- 

 corders are not available ; 



(c) EDO-UQN in depths greater than 100 

 fathoms ; 



(d) Precision Depth Recorder (PDR), 

 when available, in depths greater than 100 

 fathoms, and especially in depths beyond the 

 limit of the UQN when operating at fast 

 speed (expanded scale). 



(e) Wire sounding machine for vertical 

 cast comparisons, bottom sampling, and 

 oceanographic work. 



1-36 Echo sounder calibrations. — Most 

 echo sounders used by the Coast and Geodet- 

 ic Survey are calibrated for an assumed 

 velocity of sound in seawater of 800 or 820 

 fathoms per second. Actual velocities shall 

 be determined from serial temperature and 

 salinity observations or by velocimeter and 

 a table of velocity corrections computed (see 

 5-114 to 120). When the corrections to echo 

 soundings are less than half of one percent 

 of the depth they may be disregarded, but 

 shall be u.sed in all other cases. 



When 808 type fathometers are used, 

 phase comparisons shall be made at the be- 

 ginning of the season and shall be repeated 

 at least once each month during the season 

 (see 5-112). Bar checks shall be made and 

 recorded twice daily when sea conditions 

 permit. 



The EDO 255 may be used at a controlled 



