Page 111 control and signal building 251 



station will be dictated by the equipment available, the type of survey, and the de- 

 sired accuracy of position. In the survey of an offshore area of deep water, a lesser 

 accuracy of location may be tolerated than would be acceptable for a survey of an area 

 of relatively shoal water which may contain dangers to navigation. One should always 

 seek to attain the maximum practicable accuracy of position regardless of the 

 method selected to locate control — judgment and care should be exercised in executing 

 all phases of the procedure. 



The choice of a method of location is governed by the following factors: (a) Avail- 

 ability of existing shore control, (b) depth of water to anchor the buoy, (c) purpose or 

 use of the station, (d) availability of instrumental equipment, and (e) accuracy desired. 

 Irrespective of the method chosen, buoy stations should be selected at positions relative 

 to the shore stations from which they are to be located so that a maximum strength of 

 location will result, taking into consideration the safety of the survey vessel while 

 anchoring and locating the buoy. 



The position of a buoy station may be determined from the observed data by 

 graphic methods or by computations. In general, graphic methods furnish the required 

 accuracy for small buoy-control systems, provided accurate methods are used, but it is 

 frequently more desirable and easier to compute positions in the larger systems, par- 

 ticularly in those extending a considerable distance offshore. 



The relative advantage of each method of determining positions of buoy stations is 

 discussed in the descriptions of the various methods in 251, 252, and 253. These need 

 not be considered to exclude other methods which might be used or combinations of 

 various methods which may be employed. They are listed in the approximate order of 

 probable accuracy of the resulting position, with the most accurate method first. It so 

 happens that this corresponds in general to the order of simplicity of location and the 

 facility with which the final position is obtained. 



251. Sextant Location of a Single Buoy 



Sextant angles may be used in numerous ways to locate survey buoys and a few of 

 the more common methods are described under the following headings. Sextant angles 

 may also be combined with other observations in locating survey buoys, a few of the 

 methods being described in 252 and 253. It is good practice to repeat aU observations, 

 particularly sextant angles, as a check, and it should be understood that this is required, 

 wherever practicable, although it is not specifically mentioned in each instance. In 

 repeating observations, the interchange of angles among the observers is a good practice 

 to ensure the identification of the objects. 



For most precise results all sextant angles should be observed at, or vertically above, 

 the position desired. Manifestly this is often impracticable, but where the observations 

 are made at an eccentric location, the eccentric distance and direction should be noted. 



In locating buoys by sextant, each series of angles should be observed simultane- 

 ously and, where practicable, the horizon should be closed. Theoretically all of the 

 angles should be observed from the same point, but practically this is not feasible 

 from a ship's bridge. Generally the several observers will station themselves around 

 tli^ periphery of the bridge where nothing will interfere with their view. The result 

 is that the sum of the angles is greater than 360° by a few minutes, generally 6 or 8. 



Angles measured by sextant are not precise like angles measured by theodolite, 

 but certain precautions can be taken to increase their accuracy. Each observer should 

 station himself tovjard his angle from a central point on the bridge, the distances of all 

 observers from this point being as nearly equal as practicable. Then when the horizon 



