10 



HYDROGRAPHIC MANUAL 



(b) Contemporary surveys by a different 

 survey party ; 



(c) Contemporary surveys made by the 

 same party in different years; by different 

 methods; or from different vessels, as the 

 survey vessel and one of her launches ; 



(d) Surveys by other organizations. 



Where the hydrographic survey is contin- 

 uous in the same year, by the same method 

 and by the same survey vessel, junctions 

 between adjacent sheets may be made by 

 spacing the hydrography just as it would 

 have been spaced had the two been combined 

 in one sheet. 



1-30 Position frequency. — The position of 

 the sounding vessel must be determined at 

 intervals which will permit accurate plotting 

 of the sounding line. Obviously a knowledge 

 of the depth is useless for charting purposes 

 without a knowledge of the geographic posi- 

 tion at which the depth was measured. The 

 maximum distance between consecutive posi- 

 tions on a line should be about l^/, to IV2 

 inches on the survey sheet, regardless of 

 scale or type of control. Positions should be 

 obtained at regular intervals, the time be- 

 tween fixes being a function of speed and 

 scale (see 5-36). 



Positions should be recorded under all 

 the following circumstances when this is 

 practicable : 



(a) At the beginning and end of each line ; 



(b) When the speed of the sounding ves- 

 sel is changed appreciably ; 



(c) At all changes in course larger than 

 10 degrees; 



(d) At each detached sounding. 



The record shall show the exact clock time 

 of each position and all data necessary to 

 plot the position. 



Positions are determined by electronic 

 measurement of distances from fixed stations 

 or by the three-point fix method using sex- 

 tant angles. In the first case, the position 

 is plotted by intersection of two distance 

 arcs. Strength of the fix is determined by 

 the angle of intersection of the arcs and is 

 strongest when the angle is 90°. Intersection 

 angles less than 30° or more than 150° are 

 considered weak and should rarely be used. 



In the second case, the position is determined 

 by the intersection of circles generated by 

 loci of the sextant angles. This problem is 

 solved mechanically by use of a three-arm 

 protractor. The strength of the three-point fix 

 depends on the angle of intersection of the 

 two circles and is strongest when this inter- 

 section approaches 90". It is weakest when 

 the position is near or on a circle which 

 passes through the three fixed points. The 

 hydrographer should generally select the 

 strongest fix available (see 5-38). Sextants 

 should always be properly adjusted and elec- 

 tronic equipment correctly calibrated. 



1-31 Position numbering. — The positions 

 for each day's work on a hydrographic sur- 

 vey should be numbered consecutively. Each 

 day's work shall be identified by a letter, or 

 combination of letters assigned in alpha- 

 betical order, starting with the letter A on 

 each survey sheet (see 5-34 and 35). Capi- 

 tal letters of one color shall be used to iden- 

 tify the hydrography accomplished by the 

 ship or major survey vessel of the party ; 

 and lower-case letters to identify the work 

 of launches or other small boats of the party, 

 a different color being assigned to each sepa- 

 rate unit. 



Until the alphabet is exhausted, a single 

 letter shall be used for day letters, omitting 

 and I. After the letter Z, double letters 

 shall be used, the first series being AA, BA, 

 CA, DA, etc. 



The colors to be used to ink the position 

 numbers and day letters are purple, blue, 

 red, and green, in that order of preference. 

 Neither black nor yellow shall be used for 

 this purpose. 



The inked numbers shall be small enough 

 that they cannot be mistaken for soundings 

 on reproductions of the boat or smooth 

 sheet. 



1-32 Sounding records. — The Soundings 

 Record Book (Form 275) shall be used for 

 recording all hydrographic work (see 5-84 

 to 99) except wire-drag surveys, for which 

 Form 411 shall be used. A separate set of 

 consecutively-numbered sounding volumes 

 shall be kept for each hydrographic sheet. 



