331 HYDROGRAPHIC MANUAL PaGE 212 



for charting purposes without a knowledge of the geographic position at which the 

 depth was measured, or its relative position with reference to the land. 



The problem of hydrographic surveying is twofold, one part consisting of the 

 various methods of measuring the depths of the water and the second part consisting 

 of the measurements to determine positions in the horizontal plane. This latter is 

 called position finding, or the horizontal control of the survey. 



To determine the position of any point in a horizontal plane, at least two measure- 

 ments are required, and in all methods of control, except by astronomic observations, 

 these measurements must be to points (control stations) whose geographic positions 

 are known, or whose relative positions to one another are known. Each measurement 

 may be considered as resulting in a line of position on which the desned point is located. 

 It is obvious, then, that it is necessary to have two intersecting lines of position, straight 

 or curved, to determine the horizontal position of the point. 



This principle is applicable both to land surveying and to hydrographic surveying. 

 In the latter the two measurements may consist of azimuths, or directions, from or 

 to known control stations, which result in straight lines of position; or angles measured 

 on board the survey vessel between control stations, each of which results in a curved 

 line of position which is part of the circumference of a circle passing through the two 

 known points and the observer's position; or distances to known control points, result- 

 ing in distance arcs which are parts of circles of position. Analysis shows that, except 

 for surveys controlled by astronomic observations, all position finding consists of a 

 determination of the position of the vessel by one of the three methods or combinations 

 of two of them. 



Theoretically, the strength and accm'acy of any position determination depends 

 directly on how closely the angle of intersection of the lines of position approaches 

 90°. Practically, the accuracy of position determination obtainable in hydrographic 

 surveys decreases almost proportionately to the distance from the land, until astronomic 

 observations are used. 



331. Positions 



For proper identification, hydrographic positions shall be numbered consecu- 

 tively, starting with number 1 at the beginning of each day; and each day's work shall 

 be identified by a letter, or combination of letters assigned in alphabetical order. 



When hydrography is continuous, on a 24-houi" basis, the first position after mid- 

 night shall be considered to start a new day and shall be numbered and lettered accord- 

 ingly, except for long dead-reckoning lines (see 3311). 



3311. Day Letters 



When hydrographic surveying is on a daily basis, each day's work shall be identi- 

 fied by a letter, or combination of letters assigned in alphabetical order, starting with 

 the letter A on each survey sheet. Capital letters of one color shall be used to iden- 

 tify the hydrography surveyed from the ship or the major survey vessel of the party; 

 and lower-case letters to identify the work of the supplementary launches or vessels 

 of the party, a different color being assigned to each separate unit. These distinctive 

 letters and colors are to be used to identify the hydrographic positions throughout 

 the records and sheets. 



Until the alphabet is exhausted, single letters shall be used for day letters, omitting 

 the letters and /. After the letter Z, double letters shall be used, the first series 



