254 HYDROGRAPHIC MANUAL PaGE 128 



Small distinctive features near the offshore limits of the area of a season's field 

 work should be developed intensely with a view of possible future use as definite re- 

 coverable locations from which hydrographic surveys may be extended. 



254. Planning Buoy Control 



Buoy stations are established for use in controlling the subsequent hydrographic 

 survey, and a buoy system must be planned not only to serve this purpose conveniently 

 and adequately but so that the buoys can be accurately and economically located. 

 The first consideration is how the buoys are to be used to control the hydrographic 

 survey. On this depends to a large extent the choice of type of buoy structure and 

 anchor gear, the general method of anchoring, the distance between adjacent buoy 

 stations in line and the spacing of adjacent lines, and the accuracy of position 

 determination. 



From the increased use of buoys to control offshore surveys a number of schemes 

 have evolved, each adapted to the particular conditions encountered. The use of taut- 

 wire measuring apparatus has resulted in greater accuracy in this type of control and 

 has made the use of buoy schemes feasible for the extension of control where it was 

 otherwise impossible. 



While it is recognized that each project presents its individual problems and no 

 definite requirements can be stated which are applicable to all, a general discussion of 

 the subject and an outline of the methods that have proved satisfactory should be 

 helpful in all projects. Conditions to be particularly considered are the depth of 

 water, the character and general configuration of the bottom, the average visibility 

 for the season of the year in which the work is done, the maximum currents to be 

 encountered, the frequency of severe storms; and, if R.A.R. is to be used, the conditions 

 affecting the transmission of sound in sea water. 



2541. Spacing of Buoys in Line 



The distance between adjacent buoys in a line of buoy stations depends on the 

 prevailing visibility, the size of the buoy superstructures, and the purpose for which the 

 buoys are to be used. In general it is better to underspace rather than overspace 

 l)uoys. The delay incurred in anchoring and picking up a few extra buoys while 

 engaged on tliis class of work is usually less than that caused by inability to obtain 

 azimuths when needed. 



If the line of buoys is solely for the purpose of carrying control by traverse to an 

 offshore area to be surveyed, the buoys may be spaced at approximately a mile less 

 than the limit of average visibility of a buoy in that locality. Except in areas where 

 stable, clear atmospheric conditions prevail, the maximum limit between buoys is not 

 recommended, because of the difficulty and consequently the delay, often serious, in 

 measuring the azimuths between them during less than perfect visibility. Buoys 

 should never be spaced so far apart that there will be difficulty in seeing the far buoy 

 when azimuths are observed. In areas where exceptionally clear atmospheric condi- 

 tions pi'evail, 10 nautical miles is a suitable maximum distance between adjacent buoys, 

 provided the observers may occupy an observing station of sufficient elevation. 



The theoretical distance that a buoy of a certain height above the water surface 

 may be seen from an observing station on a vessel may be calculated with the aid of 

 Table 8, Distance of Visibility of Objects at Sea, in the American Practical Navigator 

 (Bowditch). To find the distance from which a buoy may be seen, the distance given 



