6. THE SMOOTH SHEET 



219 



Least depths on submerged dangers to navi- 

 gation and in channels are critical data 

 which should be readily identifiable on the 

 smooth sheet. To facilitate identification of 

 such features and to emphasize a critical 

 depth in an area of congested hydrography 

 it may be advisable to place a temporary 

 pencil note on the sheet. The note should 

 state the least depth and position number 

 and should be placed in an otherwise blank 

 area nearby. A short pencil arrow may be 

 drawn towards the feature but should not 

 be drawn across soundings. 



6-55 Sounding units. — The sounding units 

 to be used in various areas and depths are 

 specified in 1-38, and corresponding correc- 

 tion units are specified in 5-101. Only one 

 unit, fathoms or feet, shall be used on a 

 smooth sheet. 



On a smooth sheet whose depth unit is feet, 

 the soundings shall be plotted in integral 

 feet, except in the following places where 

 they shall be plotted to the nearest half-foot : 



(a) At important points on navigable 

 bars. 



(b) At controlling depths in dredged 

 or natural channels. 



(c) On both sides of the low-water line. 



(d) Where necessary or desirable for 

 better definition of the depth curves (Fig. 

 85). 



When plotting fractional units in feet, the 

 reduced decimal values 0.3 to 0.7 foot shall 

 be plotted as 1/2 foot. This rule also applies 

 to minus soundings. 



When plotting soundings in integral feet 

 decimals less than 0.8 foot are disregarded 

 and decimals of 0.8 and 0.9 are increased to 

 the next whole unit, except for minus 

 soundings where decimals less than —0.3 

 foot are disregarded and decimals —0.3 

 to —0.9 are treated as —1.0 foot. 



On a smooth sheet whose depth unit is 

 fathoms, soundings shall be plotted as 

 follows : 



Minus soundings and soundings in depths 

 less than 11 fathoms in fathoms and tenths. 

 In depths greater than 11 fathoms the 

 soundings shall be plotted in whole units, 

 except within areas of smooth bottom and 



gentle slope where soundings are charted in 

 feet, soundings shall be plotted in fathoms 

 and tenths to 31 fathoms and between 31 

 and 101 fathoms to the nearest half-fathom. 

 These same units shall also be used when 

 plotting soundings in the overlap area on 

 surveys plotted in fathoms which join 

 soundings plotted in feet. In all areas fathoms 

 and decimals should be plotted where neces- 

 sary for more accurate definition of depth 

 curves. 



When plotting soundings to the nearest 

 half-fathom, decimals of 0.3 to 0.7 shall be 

 plotted as V2 fathom. When plotting sound- 

 ings in integral fathoms decimals less than 

 0.8 shall be disregarded and 0.8 and 0.9 

 fathom are increased to the next whole 

 number. 



If it is necessary to convert reduced sound- 

 ings from fathoms to feet, or vice versa, the 

 conversion shall be made according to Table 

 12 and 5-123. 



In plotting fractional units for better defi- 

 nition of depth curves, it is not necessary 

 to plot all fractions at the contour depth. 

 A lightly penciled dot beside the integer can 

 be used as the plotting progresses and the 

 fractional parts penciled later on the deep 

 side of the curve. 



6-56 Size of numerals. — numerals repre- 

 senting soundings shall be 2.2 to 2.5 mm in 

 height. At this size legible photographic 

 copy can be produced even at a reduced 

 scale. A ruling pen opened to the required 

 size provides a satisfactory gage for pencil- 

 ing soundings of uniform height. Soundings 

 which mark least depths in areas of con- 

 gested hydrography should be slightly larger 

 and bolder than the above unless accentu- 

 ated by a depth curve. Such soundings are 

 among the most important in the area and 

 should be apparent on cursory examination 

 of the sheet. 



6-57 Fractions and decimals. — Figures 

 in a fraction or the decimal parts of a depth 

 unit shall not be larger than half the size 

 of the integer. The over-all height of a frac- 

 tion should not be more than 15 percent 

 greater than the height of the integer. 



