3622 HYDROGRAPHIC MANUAL PaGE 278 



In the selection, 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. 

 (6) All shoal indications rising more than 10 percent from the general depth should be ex- 

 amined. 



(c) The nature of the bottom should be considered. If it is rocky there is more likelihood of a 

 dangerous shoal and consequently it is more important that all indications be examined. If the bottom 

 is of sand or mud it is probable that the least depth may already have been determined or at any rate 

 there is little likelihood of the existence of an important danger or shoal. 



(d) The importance of the region from the point of view of navigation should be considered. In a 

 channel, harbor, or area frequently navigated every slightest indication must be examined; whereas 

 in a region little frequented for navigation the number of indications to be examined may be somewhat 

 reduced. 



3622. Local Reports of Shoals 



In all project areas pilots, fishermen, mariners, yachtsmen, local authorities, and 

 others with local knowledge should be consulted freely for the purpose of collecting 

 hydrographic information, and all reports of rocks, dangers, or shoals must be investi- 

 gated. In the past the results of such contacts have been very fruitful. Most of the 

 rocks and dangers known to such local authorities are already charted, but frequently 

 the existence of a new rock previously uncharted will be disclosed. Most of the au- 

 thorities mentioned above, except fishermen, do not know the exact locations of such 

 shoals or the exact depths on them. 



When practicable, fishermen or others should be requested to guide the hydrographer 

 to the uncharted shoals which they know. If this is impracticable, an attempt should 

 be made to verify the information from several sources before an extensive search 

 for the shoal is made. It is sometimes difficult to obtain any, or at least exact, position 

 information from fishermen, because they are not willing to divulge the locations 

 of the uncharted shoals which they know. 



3623. Visual Discovery of Shoals and Dangers 



Where the water is clear, as in the Tropics, under favorable conditions practically 

 all dangers to navigation may be discovered by keeping close watch on both sides of the 

 vessel during the survey. In addition, dangers and shoals may be disclosed by external 

 evidence such as kelp, flocks of gulls, schools of fish, riffles, eddies, upwelling, heaping 

 up of the swell, and breakers. 



The most favorable conditions for the discovery of shoals by sight are a very 

 clear sky, a calm transparent sea, and a high sun. If the sea surface is not entirely 

 calm, in order to avoid being bothered by the surface ripples, the area should be searched 

 to leeward. The sun should be high above the horizon, preferably higher than 45°, 

 and should be kept at one's back. Under certain conditions the sun may be reflected 

 from the surface of the water in a way to dazzle and prevent the observer from seeing 

 through the surface. 



The observer should station himself at as high an elevation as practicable, preferably 

 in the crow's nest. 



If the water is sufficiently clear, shoals and dangers may be readily detected 

 in shoal depths from a coloration differing from that of the surrounding bottom. 

 Rocks, especially if covered with seaweed, appear brown; coral heads and sand shoals 

 appear whitish or light green. Surface reefs and reefs almost awash appear to be 

 yellowish brown from a little distance away. Coral heads covered by about 2 fathoms 

 appear a light bright green, and shoals in depths as great as 8 fathoms appear blue 

 green in contrast with the indigo of the deeper surrounding depths. 



