352 HYDROGRAPHIC MANUAL PaGE 268 



A basic survoy is fundamental; it must bo so complete and thorough that it does not 

 need to be supplemented by other surveys, and it must be adequate to supersede, for 

 charting purposes, all prior hydrographic surveys of the area, except for some few fea- 

 tures which may be retained from the old survey (sec 9343 and 9344). It must be 

 adequately controlled by the best practicable means in current use; it must be suffi- 

 ciently intense to discover and determine the least depths on all dangers to navigation; 

 it must verify or disprove beyond question all dangers, critical depths, and other features 

 important to navigation appearing on the charts or prior surveys; it must develop sub- 

 marine features of importance in navigation; and it must provide sufficient permanent 

 control so that future revision surveys will i-equire the establishment of a minimum of 

 additional control. 



It need not cover channels and other inshore areas, recently surveyed adequately 

 on as large or a larger scale by a surveying organization such as the United States 

 Corps of Engineers, provided that survey is adequately controlled and can be correlated 

 with the basic survey and a satisfactory agreement of depths is obtained at the junction 

 of the surveys. 



The project instructions will specify any features of ] r or surveys that n.eed not 

 be verified or disproved, or superseded, as for example, bottom characteristics (3842), 

 least depths found by prior wire-drag surveys (3523), etc. 



352. Previously Known Dangers and Shoal Soundings 



All previously known or reported dangers and shoal soundings must be proved or 

 disproved by the hydrographic survey unless the project instructions specify otherwise. 

 The prior surveys and the published charts covering the project area shall be scrutinized 

 for such features. The Descriptive Report shall contain a statement that a thorough 

 comparison has been made and that the new survey is adecjuate to supersede com- 

 pletely the old data if such is the case (see 842i). When a danger or shoal indication 

 shown on a chart, siu'vey sheet, or publication is not found during the current survey, 

 the examination must b thorough enough to disprove positively its existence, and the 

 reco ds must show the kind of an examination made and its duration, and the Descrip- 

 tive Report must state why the danger is believed to be disproved; otherwise no 

 previously reported danger can be removed from any publication. (See 842A^.) 



Where a shoa' is discovered during the new survey close to a })ieviously known one^ 

 sufficient examination must be made to determine definitely whether the new feature 

 is a separate shoal, an extension of the original one, or a more accurate location of the 

 original one. 



3521. Verification of Prior Hydrographic Surveys 



A sufficient examination must be made to prove or disprove the existence of every 

 danger, shoal soimding, shoal indication, bare rock, and wreck on prior surveys within 

 the project area. It is not sufficient merety to verify their existence; their positions, the 

 least depths on the submerged features, and the elevations of the exposed ones must be 

 determined. It shoidd be borne in mind that many prior surveys were made during a 

 period of evolution of both field and office practices. Field methods were frequently 

 below present standards and control was sometimes insufficient, making coordination 

 with new well-controlled surveys difficult. 



Where the existence of a feature is disproved, or where the examination is not 

 considered adequate to disprove its existence, the Descriptive Report must contain a 

 full explanation and a recommendation for disposition of the previous data. If the 



