156 



HYDROGRAPHIC MANUAL 



by the photogrammetrist or topographer and 

 is passed on a sounding line, a note of this 

 fact shall be entered in the remarks column 

 of the sounding record with an estimated 

 distance when the rock is abeam. It should 

 be made clear that these data are not to be 

 used to locate the rock but merely as a verifi- 

 cation of its existsnce. 



If bare rocks or rocks awash shown on 

 prior surveys or on published charts are 

 found to be nonexistent, or in different loca- 

 tions, or with different elevations, a full 

 explanation shall be included in the descrip- 

 tive report, with a recommendation as to the 

 charting procedure to be followed. 



5-68 Indications of shoals and dangers. — 



There are several sources of evidence that 

 a shoal or submerged danger exists in the 

 area being surveyed and the hydrographer 

 should avail himself of all of them. This 

 information may be obtained from : 



(a) A study of soundings obtained during 

 the systematic survey. 



(b) Reports of dangers submitted by pi- 

 lots, fishermen, and yachtsmen. 



(c) Sighting of breakers, kelp, or other 

 visible evidence while sounding. 



(d) An examination of aerial photo- 

 graphs, particularly color photogrrphs, which 

 often reveal the location of shoals or rocks. 



The spacing of the systematic sounding 

 lines is designed to provide at least an indi- 

 cation of the dangers and shoals within the 

 area. Such an indication will occur as a 

 break in the continuity of the slope of the 

 bottom. A more positive evidence of the ex- 

 istence of a shoal is found where two adja- 

 cent lines of soundings contain similar indi- 

 cations. Even a slight change from the 

 average depth should be regarded as a pos- 

 sible indication of a shoal. In many localities 

 it is out of the question to examine every 

 such indication, nor is this required. The 

 hydrographer must decide which areas shall 

 be further developed, and the Chief of Party 

 must make a final inspection of the work to 

 assure himself that no additional develop- 

 ment is required. In deciding which indica- 

 tions should be developed, the hydrographer 

 should be guided by the considerations listed 



in 1-27 and his past experience in similar 

 areas. 



In all project areas, pilots, fishermen, 

 yachtsmen, and others with local knowledge 

 should be consulted freely for the purpose of 

 collecting hydrographic information, and all 

 reports of rocks, shoals, or obstructions must 

 be investigated. An attempt should be made 

 to verify the information from several 

 sources. If it is not practicable to get some- 

 one to guide the hydrographer to the un- 

 charted feature, an approximate location 

 should be obtained and plotted on the boat 

 sheet. It is sometimes difficult to obtain 

 exact information on the location of rocks 

 or shoals, and an extensive examination may 

 be required to find them. 



All members of a hydrographic survey 

 party, when not otherwise engaged, should 

 be alert to detect visible evidences of sub- 

 merged dangers. In tropical waters coral 

 banks and shoals are visible for a consider- 

 able distance when the sea is calm, the ob- 

 server is stationed well above the water, and 

 the sun is high and at the observer's back. 

 A breaker is clear evidence of an obstruction 

 or shoal, but a current eddy indicating a 

 shoal is less apparent and may not be de- 

 tected. The eddy is caused by the current 

 being disturbed in its progress by a shoal. 

 The eddy will always be seen downstream 

 from an isolated shoal, the distance depend- 

 ing on the depth of the water and the ve- 

 locity of the current. Likewise, eddies are 

 more marked when the difference in depth 

 between the shoal and surrounding bottom 

 increases. The eddies are most noticeable at 

 the last of the ebb or first of the flood tide. 



In areas where kelp grows it is one of the 

 best indications of dangers because it is gen- 

 erally associated with rocky bottom. Each 

 isolated growth of kelp must be investigated. 

 In addition to the usual data a statement 

 should be made in the record as to the value 

 of the kelp for marking the spot; whether it 

 is visible at all stages of the tide or whether 

 it tows under so as to be nearly invisible 

 at times. 



5-69 Development and examination of 

 shoals. — When it has been determined that 



