194 



HYDROGRAPHIC MANUAL 



For surveys of the greatest accuracy in 

 river and harbor work the positions of the 

 soundings are determined by theodolite or 

 transit cuts from triangulation stations suit- 

 ably situated. Two stations are sufficient to 

 determine a position, but three are advisable 

 to provide a check and to ensure that a posi- 

 tion is obtained if one observer misses his 

 observation. Ranges must be provided in 

 order that straight parallel lines may be 

 run. The soundings and position data must 

 be coordinated by time as recorded at the 

 launch and shore stations. The clocks must 

 be compared frequently and corrected if 

 necessary. The entire operation can be con- 

 trolled at the launch if radiotelephones are 

 used; otherwise the positions should be ob- 

 served on signal from the launch. Soundings 

 should be taken by pole when the depth per- 

 mits. 



Directions measured with the transits 

 must be referred to a known direction or 

 azimuth and the correct orientation with 

 respect to the initial should be verified at 

 frequent intervals. The directions should be 

 read to the nearest half-minute. 



The survey may be accurately controlled 

 by stationing one shore observer at a control 

 station with a theodolite or transit to observe 

 a direction to the vessel at each position, 

 while on board the vessel a sextant angle is 

 simultaneously measured between two ob- 

 jects so located with reference to the occu- 

 pied shore station that the circle defined by 

 the locus of the observed angle will inter- 

 sect the observed direction as nearly at right 

 angles as practicable. The positions are 

 plotted by laying off each direction line 

 from the shore station and, with a three- 

 arm protractor, plotting each sextant angle 

 so that its vertex falls on the respective 

 direction line. 



If a series of equally spaced control sta- 

 tions are accurately located along the shore 

 and successively occupied for measuring di- 

 rections to the vessel, the shore observer can 

 also direct the vessel on course when sound- 

 ing lines are run toward and away from him, 

 thus eliminating the need for ranges. 



For still greater precision, it is generally 



necessary to stop the boat for each sounding 

 at predetermined distances from the front 

 range marks. For an accurate survey in the 

 vicinity of a pier or wharf, equally spaced con- 

 trol points may be established by taped dis- 

 tances along the edge of the pier or wharf. 

 The sounding lines are generally run normal 

 to the line of control points although they 

 can be run at any selected direction there- 

 from. The boat is controlled on line by an 

 observer at the control point who marks 

 its progress with a transit or sextant, sig- 

 naling it from time to time to keep it on 

 line. 



Equally spaced soundings are generally 

 taken at predetermined distances from the 

 control points by direct measurement with 

 a line (often referred to as a tagline) , either 

 marked at equal intervals or run over a reg- 

 istering sheave. Stranded sounding wire 

 fastened to a ring or loop, marked at equal 

 distances by white rags inserted in the 

 strands, will serve admirably for this pur- 

 pose. A reel should be provided by which to 

 pay out or reel in the sounding wire. The 

 zero end of the wire may be on the boat or 

 at the control mark. If on the boat, a man is 

 stationed at the control point with the reel; 

 the boat proceeds slowly along the predeter- 

 mined line and at the proper intervals it is 

 stopped for soundings by braking the reel. 

 If the zero end is at the mark, the reel is 

 on the boat and the boat is stopped by a 

 member of the crew applying the brake. 



5-127 Track or cruise lines. — A contin- 

 uous record of soundings obtained by a ship 

 on an extended voyage en route between its 

 home port and the working grounds is called 

 a track line. The position of the line is fixed 

 by the best available means and includes 

 Loran observations, astronomic sights, and 

 dead reckoning. 



The proposed route should be laid uown on 

 a small scale nautical chart and carefully 

 transferred to appropriate sheets in the H.O. 

 3000 (1-18) series of plotting charts at a 

 scale of 1° Long, equals 4 inches. Standard 

 methods of navigation, with some refine- 

 ments, are used in an effort to follow the 

 proposed route as closely as possible. 



