S8 



HYDROGRAPHICAL SURVEYING [chap. iv. 



it is desirable to plot it instead of the actual theodolite spot, 

 the angles taken there must be corrected for the distance the 

 theodolite was from such object. 



Fig. II. 



The correction will vary according to the direction of the 

 objects with regard to the true station, as the figure annexed 

 will show. 



In Fig. 11 let A be the true station at which angles are 

 required ; B the false station ; D, E, F, H objects so placed 

 as to illustrate all positions of false and true stations with 

 regard to them. We have observed at B the angles to D, E, 

 F, H, and A, and measured the distance B A. 



Firstly. Required the angle E A D. 

 Produce B A towards C and Z. 



Now EAC = EBC + BEA 

 andDAC = DBC+BDA. 

 Subtracting we have 



EAC-DAC = EBC-DBC + BEA-BI)A 

 or EAD-EBD + (BEA-BDA). 



Secondly. 



Here 

 and 



Adding, 

 BFA) 

 or 



Required the angle E A F. 



ZAE=ZBE-BEA 

 ZAF = ZBF-BFA. 

 we get ZAE + ZAF = ZBE + ZBF-(BEA + 



EAr = EBF-{BE A+BFA). 



