92 



HYDROGRAPHICAL SURVEYING [chap. iv. 



Connec- 

 tion of 

 a Con- 

 spicuous 

 Natural 

 Object 

 >F:th a A 

 adjacent 

 to, but 

 invisible 

 from, it. 



It sometimes happens that a A is made near the top of a 

 wooded, hill, on which stands a tree which overtops all others, 

 but is difficult or impossible to identify when on the hill itself. 

 This tree is the natural object wliich will be observed from all 

 other stations, and it is therefore often desirable to connect 

 it by angle and distance with the A itself. This may be 

 effected thus : 



When at a convenient distance from the hill, fix the ship, 

 shoot up the Aj ^'^d take a careful angle between it and the 

 tree. When the bearing of the hill has changed by a fairly 

 large angle — 70° to 90° — fix as before, and again measure the 



Fig- 13- 



angle subtended by the tree and the A- The problem is then 

 quite a simple one, and as it is working down from two long 

 bases (the distance of the hill at each of the ship's fixes) to a 

 small side, it is very accurate. The easiest way to solve it is 

 by the aid of the traverse table and protraction, as follows : 



In Fig. 13, let C be the A ; D, the tree ; A and B, the 

 two ships' positions. 



C bears from A, N. 30° E. (true), taken from the plotting- 



sheet. 

 C A = 6,000 feet. ZDAC=1°00'. 

 C bears from B, N. 40° W. (true). 

 C B = 8,000 feet. ZDBC-1°30'. 

 Required True Bearing and distance of D from C. 

 Draw C E and C F± A D and B D. 



