54 NAXJTICAL CHART MANUAL 



When both the original surveys (feet and tenths) and the charts to which they have 

 been apphed show soundings in feet, it is advisable to use the original surveys to correct 

 another chart on which soundings are shown in fathoms. 



Where the soundings are plotted on an original survey in fathoms and tenths, the decimals 

 of fathoms are to be converted for charting in accordance with the following table: 



Integral Integral 



Fathom feet jiim. Vilra. fms. 



SELECTION OF SOUNDINGS 



The most important features of a chart are the soundings and depth curves by which the 

 main characteristics of the configuration of the bottom are represented. 



The least depths on shoals and the practicable channel depths must be selected before 

 spacing the surrounding depths. In narrow passages and critical areas of uneven bottom, 

 care must be taken to select enough soundings to indicate clearly the dangers and the channels 

 between them. Subject to the foregoing, the soundings in areas completely developed by the 

 hydrographic survey should be shown with uniformity and without crowding. Crowding a 

 chart with unnecessary soundings detracts from its clearness and reflects on the cartographer's 

 skill. Depth curves can be used effectively to eliminate many soundings and yet reveal the 

 bottom configuration. 



After the shoals are shown and the channels developed, the selection for the rest of the 

 chart depends on the physical characteristics of the bottom, and no hard and fast rules can 

 be given. If the slopes are gentle, the selection is simply a matter of spacing the soundings 

 as uniformly as the sm-vey will permit. If the bottom is rocky and broken, a uniformly spaced 

 selection will not reveal the true natiu-e of the ocean floor. Care must be taken not to obscure 

 a shoal sounding by placing a deep sounding too close to it. However, the deepest soundings 

 must not be neglected as they are valuable for echo-sounding navigation and for defining 

 the depth curves more clearly. Soundings of the same depth as a curve, and placed close to 

 or on the curve, serve no useful purpose on the chart and should be omitted. A curve around 

 a shoal sounding must never touch the figure. 



