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THE SMOOTH SHEET 



7762 



much greater depths and several of the shoaler depth curves are very close to the shore- 

 lines of the islands, or edges of the reefs, the shoaler curves should be omitted. 



1762. Delineation of Depth Curves 



Depth curves shall be penciled lightly as continuous lines, using a 2H pencil, and 

 using as a guide the curves shown on the boat sheet. The hydrographer will have 

 sketched on the boat sheet, many more depth curves than are required for the smooth 

 sheet (see 3533), and in doing this, certain characteristics of the submarine relief are 

 developed that may not be apparent where the prescribed curves only are drawn. 

 Careful attention must, therefore, be paid to the boat sheet when drawing the curves 

 on the smooth sheet. 



Depth curves are drawn so that each depth corresponding to the depth indicated 

 by the curve is either on the curve or within it. Wliere the curve passes between sound- 

 ings, it must be located at the correct proportional distance between the two depths. 

 Curves must be broken at sounding numerals and position dots — never continued 

 through them as continuous lines. At position numbers, however, curves should be 

 continuous, the numbers being moved slightly, if necessary. 



In comparatively shoal 

 depths where there may be 

 dangers to navigation, one 

 should always err on the side 

 of safety in drawing depth 

 curves. Wliere the soundings 

 are not spaced closely enough 

 to determine precisely the 

 position of a depth curve, the 

 curve must be drawn to in- 

 clude the part of the area in 

 which a shoal sounding is pos- 

 sible. For example, where 

 there is a shoal of less than 3 

 fathoms extending offshore, 



around which the depth curve niight be closed just outside of several 3-fathom sound- 

 ings on a given line, and there is one sounding of 3 fathoms or less on the next adjacent 

 line offshore, the one sounding must not be surrounded by a detached curve, leaving the 

 inference that there are greater depths between the 3-fathom detached sounding and 

 the inshore 3-fathom curve. On the contrary, the curve must be extended to include the 

 isolated sounding, as in figure 167. Similarly, where there is an absence of soundings 

 between two contiguous shoal areas, the depth curve must be drawn to connect both 

 shoals, rather than as a closed curve around each one, since in the latter case the infer- 

 ence would be that depths greater than the depth curve exist between the shoals. 

 (See fig. 167.) 



Without exception, depth curves should never indicate a through channel unless 

 this has been proved by the actual soundings taken. 



Over extensive flat areas it is frequently advisable to add fractional depths in 

 the vicinity of the depth curves in order to locate them more accurately (see 7713). 



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Figure 167.— Depth curves in vicinity of shoals (see text). 



