2553 



HYDROGRAPHIC MANUAL 



Page 134 



The ship positions should be solectod so that at least three cuts intersecting at good 

 angles are observed to each buoy. All buoys visible from each ship position should 

 be cut in, in order that there may be as many cuts to each buoy as possible. All the 

 angles at each ship position must be measured simultaneously and with the greatest 

 accuracy, because any slight error in position is m5,gnified proportionately in the cuts. 



The strongest cuts are those normal to the shore but their intersections are often 

 rather acute. This results in the positions of the buoys being better controlled in the 

 alongshore direction than in a direction normal to the shore; that is, adjacent buoys are 

 located with reference to one another more definitely in distance than in azimuth. To 

 remedy this, sun azimuths should be measured between the buoys, or the small horizon- 

 tal angle at each buoy between the next two buoys in line may be measured. These 

 latter should be observed at the time the buoys are anchored. Referring to figure 29^ 

 after buoys A and B have been anchored and as buoy C is being anchored the small 

 horizontal angle between buoys B and A is measured; in like manner the angle between 

 buoys B and C is measured when buoy D is anchored, and so forth until the last buoy 

 is anchored. 



The geographic positions of the buoy stations are best determined ]:)y a graphic plot 

 of the three-point fixes and cuts on au aluminum-moinited sheet. After the cuts to 



FiGURK 30. — Buoy positions strengthened hy interbuoy observations. Lower line shows interbuoy azimuths. Upper line shows 

 them adjusted to buoy cuts. (Preliminary positions marked O; adjusted positions marked HI.) 



the buoys have been plotted on the ahiminum-mounted sheet the measured azimuths 

 or the horizontal angles at the buoys arc used to plot a succession of azimuths between 

 buoys. These are plotted by means of a metal protractor on a piece of tracing paper, 

 the preliminary positions of the buoys det(u-mined by the cuts being used as a base 

 upon which to lay down the azimuths. This plot should begin at the buoy position 

 having the best intersection of cuts. With the protractor center at this position and 

 the small angle between the next two buoys in line set on the protractor, it is rotated 

 until the two arms intersect the respective groups of cuts in the best possible manner. 

 From these three tentative positions, the successive azimuth lines are plotted in both 

 directions. After the plot is completed the entire tracing is adjusted to the plotted 

 cuts and the most probable positions are accepted, always on the azimuth lines (see 

 fig. 30). 



2553. Line of Buoys Perpendicular to Shore 



A single line of buoys anchored along the axis of a narrow shoal extending offshore 

 will often furnish adeciuate control for the closely spaced lines of soundings required for 

 the development of the area. The positions of the buoys in such a line, established for 

 this purpose or for any extension of control by buoys, may be determined by simul- 

 taneous angle observations, when such accuracy is warranted. This scheme requires 

 no special distance-measuring apparatus and it may be executed by any ship that is 



