1. (JENERAL REQUIREMENTS 



work of crosslines should be run first to es- 

 tablish control for the reRular system of 

 lines. The crosslines should be very accu- 

 rately controlled, and the soundinpr equipment 

 in perfect operating condition. The sound- 

 ings should be reduced for actual tide, if 

 possible. Daily comparison, at crossings, 

 should be made. Serious discrepancies will 

 indicate an incorrect reducer, a fault in the 

 control, or faulty operation of the echo 

 .sounder and should be investigated at once. 



1-27 Indications of shoals. — A hydro- 

 graphic survey may not be considered com- 

 plete and adequate until there is reasonable 

 assurance that all dangers to navigation 

 and shoals existing in the area have been 

 found and the least depths on them deter- 

 mined. After the feature has been developed 

 by closely-spaced sounding lines, each criti- 

 cal area must be thoroughly examined to 

 determine the least depth (see 5-69). It is 

 obviously impracticable in many localities to 

 examine every shoal indication. In selecting 

 soundings to be further examined, the im- 

 portance of the locality and the types of 

 shoals or dangers to be expected must be 

 considered. Hydrographers should be guided 

 by the following considerations : 



(a) In general depths of 10 fathoms or 

 less in a navigable area, all indications 

 should be examined. 



(b) All shoal indications rising more than 

 10 percent from the general depth should be 

 examined. 



(c) The nature of the bottom should be 

 considered. If it is rocky, there is more 

 likelihood of a dangerous pinnacle. If the 

 bottom is sand or mud, there is little chance 

 that a danger exists. 



(d) The importance of the region from 

 the point of view of navigation should be 

 considered. All indications in channels and 

 harbors should be examined. In areas of lesser 

 importance, the number of examinations 

 may be reduced. 



1-28 Development of shoals. — The gen- 

 eral spacing of sounding lines should give a 

 methodical representation of the depths and 

 generalized depth curves in the area and be 



sufficiently close to give indications, at least, 

 of all banks and dangers therein. Every 

 sounding of a depth slightly less than the 

 average surrounding depths should be re- 

 garded as a definite indication of a pos- 

 sible shoal (see 5-68). The sounding lines 

 should be "split" by running intermediate 

 lines between the regular system of lines as 

 necessary to develop these indications (see 

 5-69 and 70). When the regular system of 

 lines discloses the limits of a shoal or bank, 

 a supplemental system of closely spaced 

 .sounding lines should be run in a direction 

 best suited to completely develop the feature; 

 they may be parallel with the longer axis of 

 the feature or in the form of radiating lines 

 crossing at the center of a shoal of small 

 extent. The development of the shoal indi- 

 cations furnished by the general system of 

 lines is the most important part of the work 

 and frequently the most extensive. 



In areas where the bottom is visible, there 

 is no particular difficulty in finding the least 

 depth. In other areas the least depth may 

 be determined by wire drag, by drift sound- 

 ing, or "feeling" with a leadline, especially 

 over kelp-covered rocky shoals. 



The development of a shoal and search 

 for the least depth frequently results in the 

 running of lines which cannot be smooth 

 plotted at the scale of the survey. Lines 

 which add nothing to the data already 

 recorded should be marked "not to be smooth 

 plotted" in the sounding record (see 5-25). 



1-29 Survey overlap at junctions. — An 



overlap of at least one sounding line shall 

 be made with an adjacent survey except 

 as specified below, and if the depths at the 

 junction are not in agreement, the new sur- 

 vey shall be extended into the old until a 

 satisfactory agreement has been reached 

 (see 5-16, 6-73 and 91). If a reasonable 

 extension into the other survey fails of agree- 

 ment, an investigation should be made and a 

 report submitted to the office with a request 

 for further instruction. 



The overlap specified herein shall apply to 

 the following classes of surveys: 



(a) All noncontemporary surveys ; 



