Page 139 



CONTROL AND SIGNAL BUILDING 



2562 



2562. Traverse for Three-Point Fix Control 



In an area where shoal depths extend a considerable distance offshore and the 

 liydrographic survey must be controlled by sextant fixes for a close development, a 

 system of closely spaced lines of buoys may be required. Control of this type has 

 practically replaced the tall liydrographic shore signals which were formerly used as 

 control for such surveys (see 2721). Buoys are now anchored comparatively close to 

 the coastline or at positions where strong sextant fixes are obtainable, and large areas 

 are controlled entirely by systems of lines of buoys tied to these fixed positions. The 

 present trend is toward a wider spacing between lines of buoys in areas where R.A.R. 

 methods operate efficiently, the hydrography in the mid-areas where sextant fixes 

 cannot be obtained being controlled from sono-radio buoys located at strategic places 

 in the system. 



A typical scheme is shown in figure 35. Buoys are anchored in a line from A to B normal to the general trend of the coastline 

 until the desired distance offshore is reached, then the direction of the line is changed to parallel the coastline from B to H, where 

 the direction is again changed to normal to the coastline to a shore connection at buoy I. If the sounding lines are to be run norma 1 

 to the coastline, the direction of the intermediate lines of buoys should also be normal to the coastline, as shown in the sketch, but if 

 the sounding lines are to be run parallel to the coastline, the intermediate lines of buoys should preferably parallel the line between 

 buoys B and //. 



Approximate Scale 



Figure .35. — Buoy control for three-point fixes. 



The most accurate positions are obtained if all of the distances between adjacent buoys in lines are measured with taut wire 

 However, this is not necessary, for the intermediate lines may be located as shown between buoys E and K, in which case the total 

 distance to be measured by taut wire would be 8 percent less in the scheme illustrated. The distances between buoys in the inter- 

 mediate lines may be measured by log, as illustrated in the line between buoys G and L, in which case 48 percent of the taut wire 

 would be saved. Distances measured in both directions by two well-rated taffrail logs or a submerged log are considered sufficiently 

 accurate for buoy control, provided both ends of the line are accurately located and a proportional adjustment of the closing error 

 is made. An additional line of buoys, as shown from M to N, may be located by sextant fixes from the stations in a well-located line. 

 The space between lines must be less, of course, but a second line of buoys should never be located by sextant angles from the first 

 line so located, because this cannot be done with sufficient accuracy. 



In a scheme of this type the lines of buoys from ^ to JB, B to H, and H to 7 are usually computed and adjusted as a loop traverse 

 (see 944) and each intermediate line is then computed from the traverse position of the buoy in the loop to the inshore buoy whose 

 position is fixed by sextant observations on shore stations. If taut-wire distances and sun azimuths are measured between all adjacent 

 buoys in line, the computations and adjustments may be made between each successive shore connection. The loop ABC J would be 

 computed first and adjusted, and with the adjusted position of buoy Cheld fixed, the loop CS/vT would be computed, followed in like 

 manner in each succeeding loop. This procedure is advisable when the final computations and adjusted positions are needed as the 

 survey progresses along the coast. 



