8221 



HYDROGRAPHIC MANUAL 



Page 772 



"Where corrections are entered to the nearest 1.0 foot or to the nearest 0.2 fathom or greater, the 

 range covered shall be eccentric, from one-fourth below to three-fourths above the correction applied, 

 or vice versa, according to the following tabulation: 



Range 



Corrections added 



-0. 25 to +0. 75 foot= foot 



0. 75 to 1. 75 feet = l. foot 



1. 75 to 2. 75 feet = 2. feet 

 etc 



— 0. 05 to +0. 15 fathom = fathom 

 0. 15 to 0. 35 fathom = 0. 2 fathom 

 0. 35 to 0. 55 fathom = 0. 4 fathom 

 etc. 



-0. 125 to +0. 375 fathom = fathom 

 0. 375 to 0. 875 fathom =0. 5 fathom 

 0. 875 to 1. 375 fathoms = 1. fathom 

 etc. 



8221. 



Corrections subtracted 



— 0. 25 to -1. 25 feet=-l. foot 

 - 1. 25 to - 2. 25 feet= - 2. feet 

 etc. 



-0. 05 to -0. 25 fathom = -0. 2 fathom 

 -0. 25 to -0. 45 fathom = -0. 4 fathom 

 etc. 



-0. 125 to -0. 625 fathom = -0. 5 fathom 

 -0. 625 to -1. 125 fathoms = -1. fathom 

 etc. 



Leadline Corrections 



Leadline corrections shall be entered in the ''Correction" column headed "Lead- 

 line." They result from the use of a leadline of incorrect length, whose comparison 

 with a standard has given the corrections to be applied (see 4622). The corrections 

 to be entered are derived from the entries made in Stamp No. 35 (fig. 178), at the be- 

 ginning and end of the day, and sometimes during the day. 



If the length of a leadline changes appreciably between comparisons, the correc- 

 tions shall be proportioned according to the length of time it was used for sounding. 



8222. Wire Sounding Corrections 



Wire sounding corrections are also entered in the "Correction" column headed 

 "Leadline." They result from the use of a worn registering sheave, or from the use 

 of stranded wire (see 4641). The sheave factor is entered at the beginning of the day 

 in Stamp No. 33 (fig. 176). The corrections to be entered are found by multiplying 

 the recorded soundings by the sheave factor. 



8223. Echo-Sounding Corrections 



Velocity corrections to echo soundings shall be entered in the "Correction" column 

 headed "Echo." They residt from the actual mean velocity of sound from surface to 

 bottom differing from the calibration velocity of the partictdar echo-sounding instrument 

 (see section 56) . There are two general methods by which the corrections are derived, an 

 indirect and a direct one. 



In the indirect method the corrections are obtained by numerical or graphic means 

 from observed temperatures and salinities (see 5613 and 5615). In the direct method, 

 also known as the bar check method and used for soundings in shoal depths, corrections 

 are obtained by direct comparison (see 5617). In both methods the data are used to 

 plot a correction curve, from w^hich is tabtdated the range through which each correction 

 applies (see 5614). 



In either case the corrections applicable to the various depths are entered in the 

 Sounding Record from a table, a cop}^ of wliich is included in the Descriptive Report 

 (see 8434). 



