5. HYDROGRAPHY 



5-1 Hydrographic survey defined. — Hy- 



drographic surveys by the Coast and Geo- 

 detic Survey are accomplished almost ex- 

 clusively for the purpose of publishing 

 nautical charts and related aids to mariners. 

 A hydrographic survey is not complete until 

 it meets all of the following requirements : 



(a) The area has been systematically 

 covered with accurately located depth meas- 

 urements sufficient to insure that all dangers 

 to navigation have been found (see 5-19). 



(b) All underwater features have been 

 developed including channels, shoals, banks, 

 and reefs, and the least depth determined on 

 all dangers to navigation. 



(c) The positions of all fixed and floating 

 aids to navigation have been accurately 

 determined. 



(d) Contemporary tide observations have 

 been made from which soundings may be 

 reduced to a plane of reference. 



(e) Bottom samples have been obtained 

 with sufficient frequency to reveal the gen- 

 eral characteristics of the submerged land. 



(f) Other miscellaneous operations have 

 been completed, such as locating and de- 

 scribing landmarks to be charted, accumu- 

 lation of data to be published in the Coast 

 Pilots, and measurements of magnetic 

 variations. 



Sounding is perhaps the most important 

 part of the hydrographer's duties. An ac- 

 curate knowledge of the depths is essential 

 to safe navigation, particularly in harbors 

 and their approaches were the draft of many 

 vessels is often nearly as great as the depths 

 in which they navigate. It is obviously im- 

 possible to measure the depth at every point, 

 although graphically recorded echo soundings 

 do provide a continuous profile along each 

 line. A hydrographic survey is considered 



complete when enough profiles have been ob- 

 tained to determine bottom slopes in all 

 areas. The possibility of undiscovered irreg- 

 ularities, and even dangers to navigation, 

 existing between sounding lines is always 

 present. The greatest responsibility of the 

 hydrographer is to make sure that none re- 

 mains undetected and that, when found, their 

 least depths are determined. 



5—2 Classification of surveys. — Hydro- 

 graphic surveys may be classified as basic, 

 revision, special, or reconnaissance. The 

 project instructions will specify which is re- 

 quired (see 2-4 and 2-8). A basic survey 

 must be so complete that it does not need 

 to be supplemented by other surveys, and it 

 must be adequate to supersede for charting 

 purposes all prior surveys and satisfy all 

 requirements set forth in 5-1. In addition 

 it must verify or disprove the existence of 

 charted or reported features. 



A basic survey need not cover channels 

 and other areas recently surveyed adequately 

 on an acceptable scale by other qualified 

 organizations such as the United States 

 Corps of Engineers, provided that the survey 

 can be correlated with the basic survey and 

 a satisfactory agreement of depths is at- 

 tained at the junction of the surveys (see 

 5-16). 



A special survey usually covers a small 

 area and is made for a specific purpose, such 

 as to prove or disprove the existence of re- 

 ported dangers or obstructions, to provide 

 data for harbor development, or to supple- 

 ment prior surveys for construction of a 

 large scale chart (see 5-126) . Such a survey 

 shall comply with the specifications for 

 a basic survey with respect to sounding 

 operations. 



Many leadline surveys made before the 



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