3323 HYDROGRAPHIC MANUAL P AGE 216 



of as the determination of an unknown position by the measurement of two sides of a 

 triangle when the length of the third side is known. Each control station is equipped 

 with a radio transmitter and a subaqueous sound receiver. The distances of the 

 survey ship from the control stations are determined by exploding a small bomb in 

 the water near the ship and measuring the interval of time required for the sound 

 to travel to each station. A chronograph on the ship records the explosion of the bomb 

 and the radio signals which are transmitted automatically by the control stations when 

 the sound from the bomb arrives at the respective stations. If the velocity of sound 

 through sea water is known, the distances can be computed and the ship's position 

 thus determined. 



Beyond the range of R.A.R. and in areas where such accuracy is unwarranted? 

 soundings are fixed in position by the well-known methods of dead reckoning and as- 

 tronomic observations as used in navigation, with refinements. (See 337 and 338.) 



3323. Other Methods 



In addition to the two methods used most to fix the position of the survey vessel 

 during a hydrographic survey, there are a number of other methods used in special 

 circumstances or to supplement the two more common methods. Those most fre- 

 quently used are: 



(a) Shore observations- — used solely or partly to control large-scale inshore surveys — described 

 in 334. 



(6) Estimation — where the positions are determined in whole or in part by estimated distances 

 and directions to shore detail, or control stations — described in 335. 



(c) Bearings — where the position is determined wholly by bearings from the survey vessel to 

 control stations (see 3361), or by a bearing and distance, as in the following typical examples: 



(1) One bearing and one sextant angle. 



(2) One bearing and R.A.R. distance. 



(3) One bearing and distance by vertical angle (see 33G3 and 3364). 



(4) One bearing and distance by depression angle (see 3362). 



(5) One bearing and distance by rangefinder (see 455). 



(d) Dead reckoning — where positions are determined by the ship's run from the last fixed posi- 

 tion — described in 337. 



(e) Astronomic observations — where positions are determined by observations of celestial objects — 

 described io 338. 



333. Sextant Three-Point Fixes 



Hydrography shall be controlled in horizontal position, in areas where a sufficient 

 number of suitable control objects are visible, by the well-known three-point fLx method, 

 using sextant angles, unless more precise methods are required. (See 334.) 



Position determination by two or more sextant angles taken by observers on the 

 survey vessel is the most satisfactory and commonly used method for hydrographic 

 surveys when the required objects are visible. To observe a tlu'ee-point fix, two 

 observers measure two angles simultaneously with sextants; one measures the angle 

 between the left-hand and center object, and the other the angle between the center 

 and right-hand object. The position of the vessel is then found mechanically by using 

 a three-arm protractor (see 4534). This is in effect a graphic solution of the three- 

 point problem. The advantages of the method are that all of the operations are per- 

 formed on board the survey vessel, the required data are known immediately, and the 

 position may be determined quickly. 



