Page 231 hydrography 3366 



a signal. The ship should be anchored with as short a scope of chain as practicable and 

 its position determined by observers on board whenever there is any change due to 

 change in direction of wind or current; unless, of course, it is practicable to moor the 

 ship fore and aft. The position angles by which the foremast is located and the time 

 of taking them must be noted and subsequently copied into the Sounding Record of 

 the survey. 



The foremast of a ship anchored for this purpose may be used either in conjunction 

 with other signals for three-point fixes or as an isolated buoy, the launch's position 

 from it being determined by bearing and vertical angle. In the latter case it is essential 

 that the vertical distance from some selected part of the masthead to the waterline 

 be measured accurately and all vertical angles be measured between these points (see 

 3363). 



3366. Tandem Control 



It may occasionally be desirable to have two or more adjacent sounding lines 

 run simultaneously and controlled by one ship, the other ships or launches being 

 merely referenced to it. 



When visual control is used and the visibility is such that sextant angles can be 

 measured only by observers stationed at a considerable elevation, a line run by the 

 survey ship may be controlled by three-point fixes in areas where observers on launches 

 cannot see the signals from their lower elevation. It may be advantageous to survey 

 the inshore lines around a distant reef by this method. The survey ship may follow 

 the edge of the reef at a safe distance, its course being more or less paralleled by one 

 or two launches running lines between it and the edge of the reef. In this case all 

 of the lines can be controlled best on the survey ship at each three-point fix position. 

 In addition to the two angles of the fix, a horizontal angle is measured to each of the 

 launches to provide a bearing, and a depression angle is measured to each to provide a 

 distance. 



A signal is given on the survey ship by whistle, or otherwise, at each position. 

 Generally, the inshore launch follows the edge of the reef as closely as safety will permit, 

 changing course only on the fixed positions unless changes between positions are required 

 for purposes of safety. If a second launch is used it should be instructed to maintain 

 a position about halfway between the ship and the inshore launch. 



The same method may be used to run parallel lines except that provision should 

 be made for the launch to measure and maintain a fixed distance and direction relative 

 to the survey ship. The distance may be measured by the masthead method of measur- 

 ing a vertical angle between the waterline and a definite point near the top of one 

 of the masts of the survey ship or by the use of a rangefinder. 



A similar method may be used in R.A.R., if for any reason it is desired to survey 

 with two ships in conjunction, one of which is not equipped for R.A.R. The survey 

 ship equipped for R.A.R. controls its course by R.A.R. fixes in the usual manner, the 

 auxiliary vessel following it on a parallel course and maintaining a constant position 

 from it, a little abaft its beam and at a desired distance. At each fixed position the 

 auxiliary vessel measures a compass bearing and a rangefinder distance to the survey 

 ship. When the ships are sufficiently close, the time of each fixed position may be 

 obtained on the auxiliary vessel by watching the bomber throw the bomb overboard; 

 or the auxiliary vessel may be notified by any prearranged system of signals from the 

 survey ship. The line of soundings run by the auxiliary vessel cannot be plotted at 

 the time, but at convenient intervals the position data may be radioed to the R.A.R. 



