Page 715 the smooth sheet 7712 



(o) For those offshore surveys which are entirely beyond the limits of charts whose depth unit 

 is feet; in which case the depth unit of the survey shall be fathoms. 



7712. Depth Unit in the Pacific Ocean 



The depth unit of surveys in the Pacific Ocean and bodies of water tributary 

 thereto shall be fathoms (and decimals), except — 



(a) Where the major part of a hydrographic sheet is witliin the limits of a chart whose depths 

 are in feet, that smooth sheet shall be plotted in feet. 



(6) That the survey adjacent to the shoreline shall be in feet when the sheets can be arranged, 

 without duplication, so that nearly all of the reduced depths on the inshore sheet are less than 25 

 fathoms. 



7713. Fractional Foot Units 



On a hydrographic sheet whose depth unit is feet, the depths shall be in integral 

 feet, except in the following cases, where they shall be shown to the nearest half-foot: 



(a) At important points on navigable bars. 



(b) At critical places in channels of a general depth of 42 feet or less. 



(c) The least depth on important shoals, rocks, and dangers when less than 42 feet. 



(d) In shallow enclosed waters and inside routes. 



(e) On both sides of the low-water line (see 7715). 



(/) Where necessary or desirable to define the depth curves better (see 7762). 



7714. Decimal Fathom Units 



On a hydrographic sheet whose depth unit is fathoms, soundings shall be plotted 

 as follows: less than 11 fathoms in fathoms and tenths, between 11 and 31 fathoms to 

 the nearest half -fathom, and greater than 31 fathoms in integral fathoms, except — 



Where the bottom is smooth and the slope gentle (as in parts of the Gulf of Mexico, the Atlantic 

 Continental Shelf, and the Bering Sea), and a Dorsey Fathometer or instrument of equivalent accuracy 

 is used, soundings less than 31 fathoms shall be in fathoms and tenths and between 31 and 101 fathoms 

 to the nearest half-fathom. 



7775. Minus Soundings 



Soundings that reduce to heights above the sounding datum (plane of reference) 

 are termed minus soundings, since ordinary soundings are depths below the sounding 

 datum and are considered positive numbers. Minus soundings are shown on the 

 smooth sheet preceded by a minus sign (see 7735) and should fall inshore of the low- 

 water line, such areas being bare at the sounding datum. 



In order that the location of the low-water line may be more precisely delineated 

 on the smooth sheet, the soundings in the vicinity of the line should be plotted to the 

 nearest half-foot (see 7713(g)), extreme care being taken to accompany them with the 

 correct sign. 



7716. Conversion of Depth Units 



The corrected depths in the "Reduced soundings" column of the Sounding Record 

 from which the smooth sheet is plotted are either in (a) integral feet. (6) feet and deci- 

 mals, (c) integral fathoms, or (d) fathoms and decimals. The double depth unit of 

 fathoms and feet is no longer used. 



Although the corrected depths in the Sounding Record may change from one unit 

 to another within the area of a survey or within the same Sounding Record, they are all 

 finally reduced to the unit to be used on the hydrographic smooth sheet (see 823). 

 Only one unit (fathoms or feet) shall be used on any one smooth sheet. Fractions or 

 decimals of the same unit may of course be used. 



