Page 285 hydrography 3666 



ing the boat or launch around the area being investigated. A supply of them, the type 

 depending on the area and the purpose for which they are to be used, should always be 

 at hand during the examination of shoals. 



In areas where shoals are expected and the control is weak so that it might be 

 difficult to find the shoal agam, a marker buoy should always be kept in readiness to be 

 thrown overboard to mark any shoal sounding found during the systematic survey of 

 the area. If there is any likelihood of being unable to find the marker buoy later or it 

 would be uneconomic to revisit the area, the regular soimding line should be discon- 

 tinued and the examination made at once. Otherwise the examination should be made 

 as soon as practicable because of the possibility of the marker buoy draggmg or being 

 lost if it is left for too long a time. 



3666. Record of Shoal Examinations 



Where a shoal is examined by somiding along a systematic series of lines, alMata 

 should be recorded in the Sounding Record as usual. 



Where the shoal is found or the least depth is determined through drift soundings 

 or any other nonsystematic procedure, a full report must be entered in the Sounding 

 Record of the following items when they are not otherwise evident: 



(o) The method of search used. 



(b) The length of time spent in the examination. 



(c) A statement as to whether bottom was visible or not. 



(d) The apparent area of the top and base of the shoal. 



(e) The character of the bottom. 



(/) Whether the shoal is marked by kelp, eddies, riffles, or other visible evidence. 

 (g) Sufficient additional information to enable the reviewer to determine whether the examination 

 was adequate. 



367. Development of Large Shoal Areas 



A large shoal area consisting of sand or mud where it is improbable that rocks or 

 obstructions exist may be satisfactorily developed by a system of closely spaced parallel 

 sounding lines without the necessity for further examination. It must be reasonably 

 certain, however, that the least depth, if it is less than 10 fathoms, has been determined 

 within 2 feet. 



Wliere a large shoal area may contain isolated rocks or rocky elevations a system 

 of closely spaced lines must be run over it to determine the probable location of these, 

 the most important of which must then be separately examined (see 366). It is not 

 necessary that every rock be examined, but it is essential that the shoalest one be 

 found and the least depth over it be accurately determined. If the shoal area is 

 extensive enough the least depths in several different places should be determined. 



Where an extensive foul area exists in an isolated locality where little navigation 

 is expected, such a thorough development is not required. Typical depths should be 

 determined throughout the area and around the periphery, the limits of the foul area 

 being determined accurately. The area may then be outlined on the smooth sheet 

 with a dash line within which appears the legend "foul, not thorouglily surveyed". 

 rSee also 7826.) 



368. Development of an Offshore Shoal 



For the development of areas beyond the limit of visibility of shore signals, 

 survey buoys located with reference to the shore stations shall be established, if practi- 



