2544 HYDROGRAPHIC MANUAL PagE 130 



east-west buoy lines are to be avoided wherever possible; north-south Hnes are to be 

 preferred. In a latitude and at a time of year when the sun's azimutli changes appre- 

 ciably without its reaching too great an altitude, azimuths may be observed at any 

 time during the day except near noon, and the direction of the buoy line is not so 

 important. 



If practicable, the azimuth of buoy lines should be planned so that the inclined 

 angle" may be measured by looking at the buoy direct and reflecting the sun's image. 

 The methods of observing inclined angles are discussed in 4526 and reference should be 

 made to this item so that buoy lines may be planned to facilitate observing the azimuths. 



254^4. Buoy-Control Schemes 



Survey buoys used as control for an offshore hydrographic survey are usually 

 anchored in lines throughout the area and oriented so they may be located in the 

 most accurate, effective, and economic manner. A buoy scheme must be arranged 

 so that each buoy station can be accurately located and that a maximum relative 

 accuracy can be obtained throughout the scheme. Since this type of control is com- 

 paratively expensive, no more buoy stations should be used than are needed to control 

 the hydrography and the development of an area adequately by the selected method. 



A buoy scheme should be planned which will provide effective control throughout 

 the area to be surveyed according to the method to be used and the accuracy to be 

 attained. Schemes of buoy stations depend also on the type, scale, and purpose of the 

 survey, and on the physical characteristics of the area, which differ with each project. 

 A scheme particularly adapted for the control of one area may be entirely unsuitable 

 for another of different character. A system of control for the survey of an area 

 from shoal water to deep water on the East Coast Continental Shelf would probably 

 not be satisfactory to control the survey of an extensive and critical area of shoal 

 water. The latter area would require closer development than the former and a scheme 

 of closely spaced buoy lines would be necessary. Similarly, where some uncertainty 

 in the positions of the control stations can be tolerated, there is a wider choice of 

 method of location which, in turn, extends the choice of a scheme of control. The 

 size of the area, the depth of water, the character of the bottom relief, the required 

 accuracy, and the instrumental equipment available are the principal controlling 

 factors governing the choice of a buoyscheme. 



Typical schemes of buoy control which may be employed either as control for 

 hydrographic surveys or to extend control to isolated localities are described in 255, 

 256, and 257. Combinations of the different schemes may often be effectively employed 

 in the establishment of buoy control for extensive hydrographic surveys. 



2545. Buoy Structures and Ground Tackle 



The selection of an adequate type of buoy structure and ground tackle of satis- 

 factory weight and strength is of utmost importance when preliminary plans for a 

 system of buoy control are made. The choice of a buoy structure should be made 

 with a thorough knowledge of the area to be surveyed, so that a buoy of sufficient 

 size may be selected for visibility and to support the suspended anchor cable, and the 

 counterweight required to keep it upright against the wind and water currents. The 

 different types of buoy structures are described in 281, 282, and 284. 



The anchor cable used to anchor buoys should be strong enough to withstand 

 any reasonable strain to which it may be subjected, but it should not be of such heavy 



