Page 129 control and signal building 2542 



in the table corresponding" to the height of the buoy should be added to that cor- 

 responding to the height of the observer's eye. This distance may be materially re- 

 duced by the cant of the buoy structure or by the height of waves on the horizon. 

 Where the atmosphere is slightly hazy, 5 nautical miles should be considered the maxi- 

 mum, and a comparatively clear day will be required to obtain the azimuths at this 

 distance. 



Buoys to be used as control stations for three-point sextant fixes may be located 

 5 nautical miles apart in areas where excellent visibility is prevalent but at no more 

 than 2% to 3 nautical mile intervals where slightly hazy atmosphere is to be expected. 

 In general, buoys used for this purpose should never be spaced so close as to cause 

 confusion and be mistaken for one another; neither should they be spaced so far apart 

 that fixes will be difhcult to obtain. 



Adequate R.A.R. control requires that each position be determined from at least 

 three distance arcs intersecting at good angles with one another, and sono-radio buoys 

 should be spaced in line with the expectation that they will provide such results 

 throughout the area of the survey. The spacing will depend, therefore, directly on the 

 efficiency of the R.A.R. equipment and the distance that reliable returns are obtainable. 



When sono-radio buoys are to be located by subac^ueous distances only, they should 

 be spaced well within the limits of reliable returns in order that their positions may be 

 determined from unquestionable data. 



2542. Spacing Buoy Lines 



The space between adjacent lines of buoy stations, which are to be used for sextant 

 fixes, depends on the average visibility in the locality. They must be spaced to provide 

 visible control for the area to be surveyed, and usually a spacing one and two-thirds the 

 average visibility will give satisfactory results. The spacing also depends on the size 

 of buoy structure used; a larger buoy with a larger banner or flag will, of course, be 

 visible farther than smaller buoys, especially through an exceptionally clear atmosphere. 



A slightly wider spacing may be tolerated when the sounding lines are to be run 

 normal to the direction of the buoy lines. The wider spacing is warranted only when 

 the area being surveyed is not likely to contain critical features and less rigid control is 

 permissible. 



2543. Direction of Lines 



A scheme of buoy control to be located by taut-wire sun-azimuth traverse must 

 be planned so that the direction of buoy lines permits the observation of sun azimuths 

 between the adjacent buoys. If the azimuth of the line of buoys is in the general direc- 

 tion of the sun at sunrise or sunset, inaccurate azimuths will result because the inclined 

 angle measured in one direction will be too near verticality and in the other direction 

 will be too large. In emergencies, it is sometimes possible to split the large inclined' 

 angle by measuring an inclined angle to an intermediate object and a horizontal angle 

 from it to the buoy range, but the method is generally unsatisfactory. A floating 

 object or vessel is rarely to be seen in the required direction at the proper time, and to 

 anchor a special buoy for this purpose only is an uneconomic expenditure of time and 

 effort. (See 4523.) The sun's declination, the latitude of the buoy line, and time of 

 observation are, of course, the three factors that must be considered when planning the 

 direction of buoy lines. During the summer months along the coasts of the United 

 States the azimuths should be measured in early morning or late afternoon when the 

 sun's altitude is not too high. Since the sun is near the prime vertical at these times, 



