Page 85 CONTROL and signal building 2282 



2282. Ship -Intersection Method 



There are stretches of coastHne along which control cannot be established either 

 by triangulation or by traverse, except with extraordinary difficulty and at great 

 expense. Along short sections of such a coast, control can often be established with 

 satisfactory accuracy by the ship-intersection method, provided the stations so located 

 are not used for the extension of control beyond the limited areas in which they occur. 



The method is illustrated in figure 9. A and B are triangulation stations, whose 

 geographic positions are known and which have been indisputably recovered and 

 which are intervisible. To extend control westward along the shore, a new station 

 is selected which is visible from B and from which a strategic point farther west can 

 be seen. To locate C from the line AB, observers must occupy all three shore stations 

 simultaneously. The vessel is anchored at position 1. A black-and-white target on*a 

 2- by 2-inch pole is secured to a stanchion on the flying bridge, or elsewhere, where the 

 target will be distinct and quickly detectable. 



Figure 9.— Establishing shore control by the ship-intersection method. 



The observiBg procedure is as follows: Each observer first orients his instrument on the triangulation station immediately adjacent 

 to his station; that is, A orients on B, B on A, and C on B. He then signals to the vessel by a white flag that he is ready to begin 

 observing. When all signals have been received, one long blast is blown on the whistle. This signal is the zero hour and is so recorded 

 by each of the three shore observers, whose watches have been set in agreement within a few seconds. A minute later, a flag is raised 

 on the vessel, where it is visible to the observers, to signify stand by. Thirty seconds later, it is rapidly lowered. As soon as the signal 

 flag starts down, simultaneous angle observations are made on the target by the three shore observers. One minute later, the signal 

 flag is again raised for sland by, and 30 seconds later it is hauled down for mark. This procedure is repeated for direct and reverse 

 pointings for as many positions as required. Six to eight positions with a second-order direction theodolite are considered suflScient. 



In addition to the above observations, the observer at B must measure the sum angle /IBC in order to provide data for comput- 

 ing the triangle CB 1. 



To obtain a length check on the line BC, the vessel is moved to position 2 where 

 the procedure is repeated. Best results are obtained when the sea is very calm and 

 there is a minimum of rolling and swinging of the vessel. The same identical part 

 of the pole or target must be pointed on by the three shore observers. Where long 

 lines are to be observed, the observations are facihtated and their simultaneousness 

 is ensured by the use of a sending and receiving radio set at each shore station to 

 maintain communication with the vessel. 



The control can be subsequently extended beyond C by establishing additional 

 stations and repeating the procedure. At the end of such a scheme, a connection should 

 be made to a line of the scheme of triangulation established by conventional methods, 

 in order to determine the accuracy of the locations and permit adjustment if desired. 



The advantage of this method is obvious, for the building of tall signals, the 

 transportation of the necessary lumber, and probable extensive clearing are slow and 

 expensive. Unless the importance of the area is sufficient to require triangulation by 



