7763 HYDROGRAPHIC MANUAL PaGE 732 



Depth curves or parts of curves that are not well defined, as where they are 

 dependent on one or two soundings, shall be shown as broken lines, except where close 

 inshore, in which case they shall be omitted entirely. 



The low-water line, or zero depth curve, is one of the most important curves on 

 the smooth sheet. Where it is not well defined by the soundings, data from other 

 sources as prescribed in 754 should be used for its delineation. 



7763. Inking the Depth Curves 



The depth curves are inked in the Washington Ofl&ce following the colors given 

 in table 27. The word **omit" opposite a depth in the table indicates that ordinarily 

 that curve is not to be inked. Any curves not inked shall be erased by the verifier. 



Nonstandard curves penciled by the smooth-sheet plotter to delineate submarine 

 features not sufficiently emphasized by the standard curves (see 7761), shall be inked 

 in brown on the smooth sheet by the verifier. (See also 9336 and 93426.) 



777. Inspection of Plotting 



After the positions and the soundings have been plotted and the depth curves 

 and the bottom characteristics (783) have been added, the work shall be inspected by 

 a responsible person for completeness and accuracy. In addition, the depth curves 

 and the differences in depths ,at crossings and between adjacent sounding lines should 

 be critically examined. 



The inspection should also include an examination of all critical soundings to 

 ensure that they are clear and legible; there should be no doubt, under any circum- 

 stances, as to the least depth on any rock or shoal. Neither should there be any doubt 

 about the deepest draft that can be carried safely through any channel, along any rec- 

 ommended course, or through any particular areas where passage of vessels is frequent. 



7771. Differences in Depths at Crossings 



Where excessive differences at crossings are encountered in a survey, a study of 

 the results should be made in order to discover, if possible, the source of the trouble 

 and to make such corrections as are necessary. Such differences may be due to some 

 fault in the sounding apparatus or in the method, or some error in the record. What 

 is to be considered excessive will depend on the strength of the control, the accuracy 

 of the sounding method, and the character of the bottom. It is impossible to formulate 

 a rule which will cover all of these variable conditions. One experienced in hydro- 

 graphic work will recognize, for any set of conditions and any given area, what differ- 

 ences are excessive. 



Where differences in depth are discovered, all the possible causes must be con- 

 sidered, and almost any one discrepancy may result from either an error in position 

 or an error in depth. It is particularly important to remember this, especially in 

 areas of steep slopes, where a slight displacement in the position of the sounding may 

 account for the difference in depth. (See also 563.) 



The allowable difference in any given case should not be based on a percentage 

 of the depth, but rather on the lateral displacement of the depth curves. In compar- 



