Page 269 ' hydrography- 3522 



new survey finds a minimum depth on a submerged feature which is greater than that 

 shown on the prior survey, the Descriptive Report must explain the discrepancy and 

 contain a positive recommendation by the hydrographer as to which depth should be 

 used for charting and why (see 842 AO- 



The depth curves and all critical soundings should be transferred in red to the boat 

 sheet from copies of the prior surveys (see 3234) . This will afford a comparison with 

 the prior surveys as the new survey progresses, and will also avoid the possibility of 

 overlooking soundings which should be investigated. The curves are helpful as a warn- 

 ing of the general depths to be expected and, if sounding is by handlead, the leadsman 

 can be warned when to expect deeper or shoaler depths so that he will not miss sound- 

 ings that would have to be supplied later. 



Where the general depths on a nev survey differ consistently from those on prior 

 surveys, or the least depths on shoals disagree similarly, a report should be made to 

 the Washington Office and instructions should be requested before extensive field 

 examinations are made to determine the reason. When such a discrepancy is known, 

 an examination of the records in the Office may disclose a reason for it which will 

 preclude the necessity for further extensive examination. 



3522. Verification of Charted Features 



The published charts are based on the surveys of this Bureau but in addition they 

 incorporate information from a variety of sources, such as surveys of other organiza- 

 tions, reports of vessels groimding, reports of shoal soundings taken by vessels in 

 transit, and reports of breakers. Many of these may be of doubtful accuracy and 

 reliability, but as a matter of safe policy they must be added to the charts. 



After all data which need verification have been transferred from the prior surveys 

 to the boat sheet, the latter must be compared with the largest-scale charts of the latest 

 date of the area, and any "additional similar data must be transferred to it from the 

 charts (see 3235). These data require the same examinations and reports as are re- 

 quired for features originating with prior surveys (see 3521). Unless charted data 

 from sources other than the prior surveys are quickly verified, the Washington Office 

 should be consulted regarding the facts of their origin. The records will often disclose 

 the reliability of the charted data; and if unreliable, whether the depth or the position 

 is most likely to be in doubt and if the latter, how much. 



The old surveys did not receive the close examination and review in the Washmg- 

 ton Office that surveys of today receive, and it will be found occasionally that erroneous 

 interpretation or application of the original data was made when they were applied 

 to the charts. 



3523. Prior W^ire-Drag Surveys 



Where an adequate wire-drag survey has been made in a nonchangeable area, the 

 dangers, shoals, and the least depths originating with the wire-drag survey need not 

 be verified unless this is specifically called for in the project instructions. Ordinarily, 

 the dangers and shoals originating with an adequate wire-drag survey may be con- 

 sidered final and adequate for charting without further investigation, except in a 

 changeable area. Obstructions and dangers to navigation, however, may have been 

 removed. 



If the wire-drag survey is in an area of considerable importance to navigation the 

 hydrographer must consult with the United States Corps of Engineers to learn whether 

 any of the dangers and obstructions have been removed by blasting or dredging. If 



