CHAP. XX.] FIXING POSITIONS 467 



The problem stated in Example VIII., p. 141, may be utilised Fixing 

 for fixing the position of a rock when only two objects are in °^^ ^°^^' 

 sight, the exact distance between the objects being known. 



For instance, in Fig. 114, if the rock be at B, and a boat be 

 stationed over the rook with an observer in it, another boat 

 or the ship may be anchored at A, and simultaneous angles 



being taken at B between C and D and C and A, and also 

 simultaneously angles at A between D and C and D and B, 

 by assuming a distance between A and D or B and C, the other 

 angles can be found as before, and the distance between C and 

 D then utilised to find the distance of the rock B from both C 

 and D. 



RECTANGULAR CO-ORDINATES. 



Naval surveyors are not infrequently called upon to deal with 

 trigonometrical data furnished by the Ordnance, India, or 

 Colonial Survey offices, which may be expressed in Rectangular 

 Co-ordinates. It is necessary, therefore, to understand the 

 meaning of such co-ordinates, and how they are calculated, 

 in order to plot from them. 



The following description is taken by permission from the 

 " Textbook of Topographical and Geographical Surveying," 

 by Colonel Sir C F. Close, K.B.E., C.B., C.M.G., R.E. 



