156 HYDROGRAPHICAL SURVEYING [chap. v. 



In A L M F, find M F. 



In A E M F, given M E, M F, Z E M F, find Z » M E F, 



M F E, and E F. 

 In A M F N, find M N, N F. 

 In A M N G, find N G. 

 In A F N G, given N F, N G, Z F N G, find Z « N F G, 



N G F, and F G. 



Having thus obtained tlie true bearing and distance in units 

 of" eacli station from the next, from one end of the survey to 

 the other, and having calculated the true bearing and distance 

 of the terminal stations from their observed astronomical 

 positions, the distances in units can be turned into miles and 

 all the stations plotted on the required scale. Intermediate 

 points for sounding, etc., may be shot in from the ship's 

 stations directly after the main angles are observed, or they 

 may be fixed subsequently by means of the ship when the 

 main points are plotted. 



It will be noted that the accuracy of the work depends on 

 the size of the receiving angle at each shore station between 

 the ship and the next beacon in advance of her. This angle 

 should in all cases be about 30°. In Fig. 39, the thick 

 lines show the sides entering into the calculation, and conse- 

 quently indicate those angles that are most important ; the 

 thin lines show the shots that are taken as checks, and the 

 dotted lines connect stations that are assumed to be invisible 

 from each other. 



Example XVI. — Triangulation of a coast off ivhich anchorage 

 is imp7-acticable, but on ivhich landing can be effected. 



A rapid triangulation of a coast can be carried out by means 

 of the ship under way with three observers in the foretop, 

 and four observers with theodohtes on shore at stations on 

 the coast 3 or 4 miles apart. The shore stations may or may 

 not in all cases be visible from each other ; but it is assumed 

 that occasionally at least the trend of the coast is such as to 

 permit of two stations being in sight from one another, 

 and the more frequently this occurs the more accurate is the 

 work likely to be. 



