Page 135 



CONTEOL AND SIGNAL BUILDING 



2553 



equipped to handle buoys, but at least four observing parties are required to measure 

 simultaneous sextant angles. 



Buoys are anchored in a line from^d to E, as in figure 31, spaced so that they will 

 be intervisible. The initial buoy A should be anchored with reference to the shore 

 stations so that strong figures will be formed between it, the shore stations, and the 

 inshore temporary ship positions (marked 1 and 5); and the successive buoys in line 

 should be anchored so that the strongest practicable figures can be carried forward to 

 maintain a maximum accuracy of position determinations. In general, buoys equally 

 spaced in an approximate straight line will provide the strongest figures, and in no 

 circumstance should the intervals between buoys differ greatly. Equilateral figures 

 will do\ibtless give the strongest position determinations. While the buoys are being 

 anchored, dead-reckoning data should be observed so that the approximate positions 



^'--:^--::SU:::^ 



Approximate Scale 



f/3 ..X \ \ ,-'-J^/ 



X, >^ / \^,'' / I 



10 Miles 3 ~~'^-\'j'< --''''\ // 



FiouRE 31.— I-ine of buoys located by simultaneous sextant angles. 



of the buoys may be plotted and used to determine where the ship stations should be 

 to form the strongest figures when the observations are made. 



To obtain accurate positions of buoys by this method, the angles in each triangle 

 in the scheme must be observed simultaneously and observations should be made 

 on both sides of the line of buoys so that a check may be obtained on the positions 

 determined. 



The illustrated scheme shows the required observations and the method to extend the observations offshore, two methods of making 

 the initial obs ervations being shown. The first is shown with the ship at position 1 and illustrates the method of obtaining observations 

 where three shore stations are available. The ship occupies position 1, a shore observer occupies station i' with a theodolite, and two 

 launches with observers are sent to occupy buoys A and B. Simultaneous observations are made at all four stations by the visual 

 method described in 2282, or by any other means available. The directions marked 1 in figure 31 are obtained simultaneously at each 

 station occupied and these observations furnish data so that the position of the ship at position 1 may be computed by the three-point 

 problem, the position of buoy A computed with a check, and that of buoy B computed from the single triangle; these computations 

 furnish the length and azimuth of the line AB which arc then used to compute the triangles in carrying the computations ahead. 

 After the observations at the first positions are completed, the ship moves ahead to position 2 and the shore observer at station Fis 

 taken in a launch to occupy buoy C. Simultaneous observations are then taken as indicated by the directions marked 2, and in turn, 

 observations through the scheme are made in' this manner for each succeeding position of the ship, the launch observer at the inshore 

 buoy always moving ahead to the next unoccupied buoy position. 



If the shape of the shoal area requires it, an additional buoy F can be cut in without bemg occupied, the observations illustrated 

 providing for a check on its position. 



Where only two shore stations, as Fand Z in figure 31, are available from which to obtain the initial observations an observer 

 must occupy each of the shore stations as illustrated for the ship at position 5. Observers are stationed at buoys A and B as befoie 

 and with the ship at position 5, simultaneous observations are taken as indicated. Succeeding observations are obtained as described 

 above until observations in the entire scheme are comjileted. 



