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CHAPTER 3. HYDROGRAPHY 

 31. GENERAL STATEMENT 



Nautical charts which mariners use for the safe and expeditious navigation of their 

 ships along coasts and into harbors are based chiefly on hydrographic surveys. The 

 surveys must not only discover and locate all dangers within the area but must deter- 

 mine the depths of the water and where these depths are measured. For an accurate 

 and adequate nautical chart it is necessary to collect and compile much more material 

 than appears on the finished chart. 



A complete hydrographic survey of an area may be described briefly as consisting of: 



(a) A systematic coverage of the area with depth measurements sufficient to ensure that all 

 dangers to navigation have been found. 



(6) A development of all underwater features of special significance to navigators, such as chan- 

 nels, reefs, banks, shoals, and characteristic submarine features; and the determination of the least 

 depths on all dangers to navigation. 



(c) The location of the soundings, dangers, and submarine features so that they can be charted 

 correctly with reference to the adjacent land features, or by latitude and longitude. 



(d) Contemporary tide observations from which the soundings may be reduced to a reference 

 plane, and often for the determination of this plane. 



(e) Supplemental operations to locate and obtain the descriptions of numerous features, such 

 as rocks, reefs, wrecks, aids to navigation, and landmarks, that must be charted and described in 

 the Coast Pilots published by the Bureau (see 91). 



Sounding is perhaps the most important, as well as the most arduous, part of the 

 hydrographer's duties. An accurate knowledge of the depths is essential to safe navi- 

 gation, particularly in harbors and their approaches where the drafts of vessels are often 

 nearly as great as the depths in which they navigate. 



A topographer can examine visually the terrain and features which he is mapping ; 

 but the submarine relief is hidden from the hydrographer and its form and features can 

 be deduced only from depth measurements systematically spaced over the area being 

 surveyed. It is obviously impracticable to measure the depth at every point, and a 

 hydrographic survey is considered adequate when enough depths are measured so 

 that the slope of the bottom between them can be assumed to be uniform. Of course, 

 there is no justification for such an assumption; however, no better method has yet 

 been devised than to cover an area with a system of evenly spaced sounding lines. 

 Echo sounding, particularly when recorded graphically, does provide a continuous 

 record of the slope along each line. 



The possibility of undiscovered irregularities, and even dangers to navigation, 

 existing between sounding lines is ever present and the gravest responsibility of the 

 hydrographer is to make sure that none remains undetected and that, when found, their 

 least depths are determined. 



311. Depth Measurements 



3111. Accuracy of Depth Measurements 



The depths of the water shall be measured with the greatest accuracy consistent 

 with efficient progress. No depth-measuring instrument or method shall be used to 

 sound over relatively even bottom or in critical depths which does not measure depths 



