5. HYDROGRAPHY 



143 



ferred to a base line which is a multiple of 

 400. The value of the baseline is noted at 

 each change by such an expression as 400 

 plus, 800 plus, etc., (see 3-83). 



5-32 Care of fathograms. — The fatho- 

 gram is the original record of the soundings 

 and must be carefully preserved (see 1-37). 

 Fathograms can be stored on spools but it 

 is preferred that they be folded in accordion 

 folds and filed in manila envelopes, or bellows 

 files. The envelops or files shall be labeled 

 to show their contents by sheet number, 

 sounding vessel, and date or day letter of 

 each fathogram. 



Fathograms shall be carefully packaged 

 and forwarded by registered mail or express. 

 They shall not be transmitted in the same 

 shipment with the smooth sheet or sounding 

 record books (see 7-24.). 



5—33 Position fixing. — In order that a 

 sounding may be charted in its correct lati- 

 tude and longitude or in proper relation to 

 the adjacent shore, it is necessary to deter- 

 mine the position of the sounding vessel at 

 frequent intervals. If the vessel is proceeding 

 at a constant speed, and on a fixed course, 

 the soundings between positions can be 

 spaced and plotted quite accurately. The 

 position is generally determined by one of 

 two methods: (1) Three-point sextant fixes; 

 or (2) simultaneous measurement of two 

 distances to shore stations. Occasionally a 

 position may be expressed as an estimated 

 distance and direction from a nearby signal 

 or other known point. 



5-34 Position identification. — For pro- 

 per identification, . hydrographic positions 

 shall be numbered consecutively, starting 

 with number 1 for each sounding vessel at the 

 beginning of each day, and each day's work 

 shall be identified by a letter, or combina- 

 tions of letters assigned in alphabetical or- 

 der (see 1-31). When hydrography is contin- 

 uous, on a 24-hour basis, the first position 

 after midnight shall be considered to start 

 a new day and shall be numbered and let- 

 tered accordingly, except for long dead 

 reckoning lines which are often given one 

 day letter for each line. 



When a sounding line is continued from 

 one sheet to another and the last position on 

 one sheet is the same as the first position 

 on the other, the position shall be identified 

 by number and letter appropriate to each 

 sheet. 



5-35 Day letters. — Each day's hydrog- 

 raphy shall be identified by a letter assigned 

 in alphabetical order starting with the let- 

 ter A on each sheet and omitting the letters 

 and I. Until the alphabet is exhausted, 

 single letters shall be used. After the letter Z, 

 double letters shall be used, the first series 

 being AA, BA, CA, etc., the second series 

 being AB, BB, CB, etc., and likewise for 

 successive series as needed. 



Capital letters of one color shall be used 

 to identify the hydrography surveyed from 

 the ship or major survey vessel of the party; 

 and lower case letters to identify the work 

 of the launches or supplementary vessels of 

 the party, a different color being assigned 

 to each unit. A separate series of day letters 

 shall be used to identify the work of each 

 sounding vessel in the records and on the 

 sheet. The colors to be used to ink the posi- 

 tion numbers and day letters are purple, blue 

 red, and green in that order of preference. 

 The day letter in the sounding record shall 

 be recorded in the color assigned that unit 

 (see 1-31). 



5-36 Frequency of positions. — The fre- 

 quency of positions along a sounding line is 

 influenced by several factors: the scale of 

 the survey ; line spacing ; speed of the survey 

 vessel; conditions of wind and current; and 

 type of control being used (see 1-30). The 

 maximum distance between successive posi- 

 tions on a sounding line should be about IV2 

 inches on the survey sheet, regardless of the 

 scale. Positions should be obtained at regular 

 intervals as this will be of advantage in 

 plotting and spacing soundings and will aid 

 in detecting errors in plotting. 



When sextant fixes are used to control the 

 (hydrography, positions must be obtained 

 more frequently were there is diflficulty in 

 keeping on line due to currents or where 

 lines must be closely spaced. Positions may be 



