Page 105 control and signal building 2432 



if such accuracy is warranttd or if there is much distortion in the boat or smooth sheet. 

 The three-pomt problem may be compuled on Form 655; the method is described in 

 detail on pages 98 to 100 in Special Publication No. 145 (see also 2285). 



2432. Station Cut In From Vessel 



The method which is most convenient and which is normally used by the hydro- 

 graphic party is to fix successive positions of the vessel by three-point sextant fixes, 

 from each of which a simultaneous sextant cut is measured from a control station to 

 the new station. The vessel should be stationary at each three-point fix and preferably 

 at anchor. Accurate results cannot be obtained from a vessel underway. The repeti- 

 tion of each set of observations at a ship station serves as a check on the accuracy and, 

 if the angles observed are interchanged among the observers, serves to verify the 

 identification of the objects. 



To measure the necessary sextant angles simultaneously, at least three observers 

 are required; two to measure the angles of the fix, and the other to measure the angle 

 from one of the control stations to the new station or object to be located. The ship 

 or launch positions must be carefully selected so that strong fixes can be obtained and 

 so that the cuts will give a good intersection at the new station. A minimum of three 

 cuts is required, which may be considered sufficient if they intersect in a point when 

 plotted graphically. Additional cuts are desirable. The more nearly the best cuts 

 approach an intersection of 90° the stronger theoretically will be the location of the 

 station, other intermediate cuts serving as checks. 



2433. Station Cut In From Shore Statioris 



A third method which may be used is to occupy three or more control stations 

 with a sextant, observing at each an angle from another control station to the new 

 station. The occupied stations should be selected to provide intersections at the new 

 stations as required in 2432. 



244. Accuracy of Location 



Hydrographic stations located for the control of inshore hydrographic surveys 

 shall be located with an accuracy approaching that required for stations located by 

 topography, since in effect they are to supply deficiencies in the topographic control. 

 For hydrographic stations to be used in controlling the offshore hydrography, a lesser 

 accuracy may be tolerated. In addition to any other requirements, control stations 

 located by any methods whatsoever should be of such an accuracy that no appreciable 

 errors will be projected into the positions of the soundings at the scale on which the 

 work is protracted. 



While engaged in sounding, the hydrographer shall watch particularly for any dis- 

 crepancies in successive positions caused by changes of fix. If such are detected an 

 immediate effort must be made to determme whether or not they are caused by errors 

 in the positions of one or more of the signals used and, if so, to determine which signals. 

 The facts shall be reported immediately to the Chief of Party who shall take such steps 

 as are practicable to verify the locations and relocate such signals as are necessary. 

 If the questionable stations were located from a planetable traverse between control 

 stations, the entire traverse shall be rerun (see 232). 



