37 HYDROGRAPHIC MANUAL PaGE 286 



cable, ill order that details of the area may be plotted in their proper positions and the 

 area be completely and economically covered. Where this is practicable, the investiga- 

 tion of the shoal area is carried out, using the survey buoys for signals, just as any other 

 similar area would be surveyed. Where this is impracticable, two other methods may 

 be used. 



Where an offshore shoal of small area and of shallow depth must be developed 

 from a launch or small boat where three-point fixes cannot be used, a buoy with a tall 

 mast may be anchored with a short scope near the center of the shoal. The height of 

 the top of the mast above the waterline must be known and the method can be used 

 only w^ien the counterweight is heavy enough and the sea calm enough so that the mast 

 remains almost vertical. After the buoy has been located, lines radiating to and from 

 it may be run by compass courses, the distance from the buoy at each fix being deter- 

 mined by measuring the subtended vertical angle between the top of the mast and the 

 waterline (see 3363). The deviations of the compass must be accurately known. 

 (See also 3365.) 



Where the shoal is extensive and the depths on it are not dangerous, one survey 

 buoy may be anchored at or near its center and located. The area may then be sys- 

 tematically covered with the survey vessel by a series of lines radiating from the buoy, 

 the positions close to the buoy being referred to it by compass bearings and range- 

 finder distances and the positions farther away by bearings and depression angles or 

 by dead reckoning and log distances from the buoy. 



To develop closely an offshore area where shore control is visible but is so distant 

 that it cannot be plotted within the limits of a large-scale sheet, circle sheets may be 

 used (see section 37). 



37. LARGE-SCALE OFFSHORE SURVEYS 



In offshore hydrogi'aphy using three-point fix control it is frequently desirable to 

 develop portions of the area on a larger scale than that used for the general area, as 

 for example, extensive shoals which can almost never be satisfactorily developed on 

 a small scale. Even if a large-scale sheet can be constructed to include the required 

 control stations, it will usually be found that protractor-arm extensions are required, 

 making it difficult to plot positions quickly and accurately. Any distortion of the 

 boat sheet further complicates the problem. But if a sheet of the desired scale cannot 

 be constructed to include the required control stations, the conventional metho'd of 

 plotting three-point fixes certainly cannot be utilized. To obviate these difficulties, 

 circle sheets can be used on which fixes can be plotted without the use of a protractor. 

 A wider application of this method is strongly recommended. 



Simply stated, the method consists in drawing on the boat sheet or smooth sheet, 

 intersecting systems of arcs of circles, .each circle corresponding to the locus of some 

 angle between two control stations. A position is then plotted at the intersection of 

 the loci of the two observed angles, each locus being found by interpolation between 

 the arcs drawn on the sheet. 



Although considerable work is involved in placing the arcs on a sheet originally, 

 the ease with which fixes can be plotted and the increased accuracy of the results 

 obtained more than compensate for the time spent, especiafiy where numerous fixes 

 observed to the same stations are to be plotted. 



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