25 HYDROGRAPHIC MANUAL PaGE 110 



visibility of shore control, or of developing shoal areas out of sight of land. This type 

 of control has increased in importance with the need for carrying surveys farther and 

 farther offshore, and with the necessity for controlling such surveys more accurately 

 than was possible by dead reckoning or astronomic methods. Until recently the areas 

 in which buoy-controlled surveys were practicable were limited by the necessity of 

 anchoring buoys in comparatively shoal depths. The Coast and Geodetic Survey 

 has recently adapted ground tackle equipment for use in deep water. With it buoys 

 have been anchored in depths as great as 1,300 fathoms and the same equipment can 

 probably be used in depths of 2,000 fathoms. (See 2834.) 



In areas of comparatively shoal depths, in which dangers to navigation may exist, 

 a thorough hydrographic development is necessary. In such areas the survey is best 

 controlled by sextant fixes for position determination. Such surveys must be extended 

 a considerable distance offshore along much of the Atlantic Coast and even more 

 extensively in the Gulf of Mexico. In these areas the approved practice is to control 

 the hydrography as far offshore as practicable by sextant fixes on shore objects, and 

 then to establish a system of buoys to which sextant angles are observed to control the 

 remainder of the required survey. 



Offshore surveys in areas of moderate depth, beyond the visibility of shore stations 

 or where the development does not require sextant fixes on survey buoys, are controlled 

 by R.A.R. methods or accurate dead reckoning or a combination of the two. A system 

 of buoys may be used in each case to provide greater accuracy and to coordinate the 

 survey as a whole. The buoys may be sono-radio buoys or survey buoys, or both, at 

 stations of known position and at which the dead-reckoning lines are started and 

 ended. Frequently shoals, small or extensive in area, are found far oft'shore on which 

 close development is required in order to determine the least depths. If the area is 

 extensive, a system of buoys is required to provide the control. If the shoal is small, 

 a single buoy located near its center will suffice if bearings to it can be obtained from 

 any part of the area. If R.A.R. methods can be used, distances obtained from a sono- 

 radio buoy near the center of the shoal, combined with visual bearings, provide an 

 excellent method of developing the area. Short distances to a buoy may be determined 

 from depression angles observed to the waterline of the buoy, provided the height of 

 eye of the observer is appreciable and accurately known (see 3362). 



Other uses of buoys are to extend or supplement shore control. A buoy traverse 

 may be used to carry control along an inaccessible coast where triangulation is imprac- 

 ticable. Buoys may be used to furnish control for inshore hydrographic surveys along 

 a coast where it is impossible to make landings to build and locate shore stations. 

 Also a buoy traverse may be used to extend control across a body of water to an island 

 or shoal area to which it cannot be extended by conventional land methods. 



In the establishment of the various systems of buoy control there are many details 

 which are common to several, and to avoid repetition this section treats first of the 

 most simple case, the location of a suigle buoy, and expands to the most complex, the 

 location of an extensive buoy system by traverse and acoustic distances. 



Any system of buoys used for the control of offshore hydrography beyond the 

 visibility of shore stations must be connected with established shore control or located 

 by astronomic observations. A buoy station within the visibility of shore stations 

 may be located almost as accurately as a shore station by various combinations of 

 sextant angles and azimuths. There are various methods of locating a station beyond 

 the visibility of shore stations, the accuracy attained being dependent on the method 

 and instrumental equipment used. The method selected to locate any offshore buoy 



