3343 HYDROGRAPHIC MANUAL PaGE 224 



shore observations should be noted to the nearest second and recorded in the Sounding 

 Record on the vessel and in notebooks at each shore station together with the respective 

 observations. 



The directions measured with the theodolites or transits on shore must be referred 

 to a known direction or azimuth and the object used for orientation should be verified 

 and noted at the beginning of each page of the record by recording a pointing on the 

 reference object. For plotting the positions of the soundings graphically, it is advan- 

 tageous, although not essential, that the control stations be within the limits of the 

 survey sheet; and it is convenient, although again not essential, if the reference station 

 used for orientation is to the shore observer's left as he views the area being surveyed. 



Directions measured with theodolites or transits should be read and recorded to the 

 nearest half-minute; additional accuracy is unwarranted for graphic plotting. 



The disadvantages of the m.ethod are that the hydrographer cannot plot the posi- 

 tions of the vessel as the work proceeds and that ranges must be provided for controlling 

 the course of the vessel; otherwise a straight line of soundings cannot be run. 



3343. Surveys Controlled Partly on Board and Partly Ashore 



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 objects so located with reference to the occupied shore station that the circle defined 

 by the locus of the observed angle will intersect the observed direction as nearly at 

 right angles as practicable. (See 2522.) 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 stations are accurately located along the shore 

 and successively occupied for measuring directions to the vessel, the shore observer 

 can also direct the vessel on course with his instrument, if sounding lines are run toward 

 and away from him, thus eliminating the need for ranges. 



3344. Soundings Located by Measured Distances 



For still greater precision and on larger scales, e.g., 1:1 ,000, 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 control 

 points may be established by taped distances 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 therefrom. The boat is controlled on line by an 

 observer at the control point who marks its progress with a transit or sextant, signaling 

 it from time to time to keep it on line. 



Equally spaced soundmgs are generally taken at predetermined distances from the 

 control points by direct measurement with aline, either marked at equal intervals or run 

 over a registering 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 

 purpose. 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 predetermined 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. All lines must be run away from the . 



