174 



HYDROGRAPHIC MANUAL 



= fathom. 

 = 0.2 fathom. 

 — 0.4 fathom. 



0. fathom. 

 0.5 fathom. 

 1.0 fathom. 



fathoms. 



Range 

 Correctitm* added 



-0.25 to 0.75 foot = foot. 



0.75 to 1.75 feet = 1.0 foot 



1.75 to 2.75 feet = 2.0 feet, 

 etc. 

 -0.05 to 0.15 fathom 



0.15 to 0.35 fathom 



0.35 to 0.55 fathom 



6tC. 



-0.125 to 0.375 fathom = 



0.375 to 0.875 fathom = 



0.875 to 1.375 fathom = 



etc. 

 -0.25 to 0.75 fathom = 



0.75 to 1.75 fathoms = 1.0 fathoms. 



1.75 to 2.75 fathoms — 2.0 fathoms. 



etc. 

 -0.5 to 1.5 fathoms = fathoms. 



1.5 to 3.5 fathoms = 2.0 fathoms. 



3.0 to 5.5 fathoms = 4.0 fathoms. 



etc. 

 -1.0 to 3.0 fathoms = fathoms. 



3.0 to 7.0 fathoms — 4.0 fathoms. 



7.0 to 11.0 fathoms = 8.0 fathoms. 



Corrections avhtracted 



-0.25 to -1.25 feet = -1.0 foot. 

 -1.25 to -2.25 feet = -2.0 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 = -2.0 fathoms. 



etc. 

 -0.25 to -1.25 fathoms = -1.0 fathoms. 

 -1.25 to -2.25 fathoms = -1.0 fathom. 



etc. 

 -0.5 to -2.5 fathoms = -2.0 fathoms. 

 -2.5 to -4.5 fathoms — -4.0 fathoms. 



etc. 

 -1.0 to -5.0 fathoms = -4.0 fathoms. 

 -5.0 to -9.0 fathoms = -«.0 fathoms. 



etc. 



In depths over 101 fathoms, recorded 

 soundings shall be corrected in intervals of 

 1.0, 2.0, 5.0 and 10.0 fathoms when the 

 corrections are entered in units of 0.5, 1.0, 

 2.0 and 4.0 fathoms, respectively. If the 

 algebraic sum of the individual corrections 

 does not add up to a multiple of one of the 

 intervals given above, it shall be arbitrarily 

 changed to the nearest multiple. When half- 

 way between two intervals, it shall be 

 changed so as to increase negative correc- 

 tions and decrease additive corrections. Thus 

 when soundings are recorded to 1.0-fathom 

 intervals, and the algebraic sum of the cor- 

 rections is —3.5 fathoms, a —4.0 fathoms 

 correction shall be made to each sounding. 

 When soundings are recorded to the nearest 

 5-fathom interval and the algebraic sum of 

 the corrections is —2.5 fathoms, a correc- 

 tion of —5 fathoms shall be applied, but a 



correction of -|-2.5 fathoms shall be arbitra- 

 rily reduced to 0. 



When no corrections are to be applied to 

 the recorded soundings, the word "Same" 

 shall be written at the head of, or in, the 

 FIELD column under the heading RE- 

 DUCED SOUNDINGS, and the soundings 

 shall not be transferred to the latter column. 



In all areas in depths over 101 fathoms, 

 corrections shall be omitted where the alge- 

 braic sum of the tide correction and other 

 corrections, excluding velocity corrections 

 (see 5-114) is less than half of one per- 

 cent of the depth. In offshore coastal areas, 

 tide corrections shall be omitted in depths 

 greater than 101 fathoms and shall not be 

 used in computation of the total algebraic 

 sum of corrections. 



5-102 Tide reducers. — Except as stated in 

 5-101, all soundings must be corrected for 

 the height of the tide above or below the 

 tidal datum plane adopted for the area. For 

 the Atlantic Ocean and Gulf of Mexico 

 the plane of reference is the mean of the 

 low waters (MLW) ; and for the Pacific Ocean 

 the plane of reference is the mean of the 

 lower low waters (MLLW) (seel— 45). 



The tide reducers are derived from auto- 

 matic tide gages at standard stations, or 

 from automatic tide gages established in the 

 project area for this purpose. Occasionally 

 the reducers may come from observed tides 

 (heights read from a staff) (see 2^9 and 

 50). 



Predicted tides are generally used to re- 

 duce soundings for plotting on the boat sheet 

 (see 5-63) but the final reducers to be en- 

 tered in the sounding record reflect the ac- 

 tual rise and fall of the tide as nearly as 

 possible. For new tide stations the mari- 

 grams must be sent to the Washington Office 

 where the plane of reference is determined. 

 In such cases the hourly heights must be 

 scaled for the periods when soundings were 

 recorded so that the tide curve can be re- 

 constructed at a later date. 



Three printed forms are available for re- 

 construction of tide curves as follows : 



Form 114 for reducers entered to 0.2 foot. 



Form 115 for reducers entered to 0.5 foot. 



