3112 HYDROGRAPHIC MANUAL Fage 192 



less than 11 fathoms accurately within one-half foot and greater depths within 1 per- 

 cent, unless specifically authorized by the Director. 



In rapidly changmg depths and over irregular bottom the requirements may be 

 lowered to 1 foot for depths,less than 11 fathoms, and 2 percent in greater depths. It is 

 recognized that a depth measured in a submarine valley or on a steep submarine slope 

 by a nondirective echo-sounding instrument may be less than the depth vertically 

 under the survey vessel. (See 563.) If it is important to know the vertical depths in 

 such areas, the project instructions will specify that wire soundings supplement the 

 echo souD dings. 



3112. Depth Units 



All depths shall be measured, and the results recorded, either m fathoms and 

 tenths or in feet and tenths. The double unit, fathoms and feet, shall no longer be used. 



Only one depth unit shall be shown on each hydrographic survey sheet — the depth 

 unit to be used depending on the general and specific locality, the depths, and which 

 unit is used on existing charts (see 771 to 7712). Tliis does not require that the depths 

 for each hydrographic sheet be measured and recorded in only one unit, but when this is 

 practicable the reduction of the records will be facilitated (see 8143). 



The following rules shall be followed in measuring and recording depths insofar 

 as practicable: 



(a) All depths of 11 fathoms or less — in feet (and decimals). 



(b) Echo soundings with the Dorsey Fathometer for a hydrographic survey to be plotted in 

 feet — IN FEET (and decimals). 



(r) Echo soundings with the 808 Fathometer or the Hughes il/<Sl2D for a hydrographic survey 

 to be plotted in feet — in feet (and decimals), except for (d). 



(d) When shoal-water graphic-recording echo-sounding instruments, which can be operated to 

 record in either feet or fathoms, are used in areas of irregular bottom, the first phase in feet shall be 

 used to its limit (about 50 feet) but fathoms shall be used for greater depths. 



The purpose of this rule is to avoid the numerous changes in phase that would be required if all 

 depths were recorded in feet, to the extent that confusion might result and the record be unnecessarily 

 difficult to interpret. 



(e) All depths measured by other echo-sounding instruments, described in this Manual as used by 

 the Coast and Geodetic Survey — in fathoms (and decimals). 



(/) Handlead soundings interspersed with echo soundings — in the same unit as the echo soundings. 



Whenever a change of depth unit occurs during the day's work, the change must 

 be emphatically indicated by writing the new unit in the "Soundings" column when the 

 change occurs, and appropriate notation must be made in subsequent column headmgs 

 (see 8143). 



3113. Approved Range for Methods and Instrument 



The most accurate method and instrument available, whose uses are practicable, 

 shall be used in all depths less than 1 1 fathoms ; but care must be taken not to extend the 

 use of a method or instrument particularly adapted to shoal inshore areas beyond its 

 practicable limit, where other methods or instruments are more suitable. 



3114. Approved Methods of Depth Measurement 



No method of sounding or type of sounding apparatus, not approved by the 

 Washington Ofiice, shall be used in hydrographic surveying, except for experimental 

 purposes. 



The three general methods, now approved, for measuring depths are ; (a) , indirect 

 measurement by sound (echo sounding) treated exhaustively in chapter 5, (b) 



