CHAIN SURVEYING 



31 



To Determine the Distance to an Inaccessible Point. Let 



it be required to determine the distance from the point B to 



an inaccessible point P, Fig. 4. 



Measure BC in any convenient 



direction and run a line A'D' 



parallel to BC. Measure AD, 



the distance between the points 



where the lines PB and PC 



intersect A'D'. Measure also 



AB. Then, 



ABXBC 

 ~ AD-BC FIG. 4 



EXAMPLE. If, in Fig. 4, BC= 100 ft., AB = 52A ft., and AD 

 = 124.2 ft., what is the distance BP? 



SOLUTION. Substituting these values in the preceding 

 equation, 



52.4X100 



BP- 



124.2-100 



= 216.5 ft. 



To Determine the Distance Between Two Points Invisible 

 From Each Other. Let it be required to find the distance 

 between two points A and B, Fig. 5, that are invisible from each 





FIG. 5 



other. First run a random line AD' in such a manner that it 

 will pass as near B as can be estimated. From B drop a per- 

 pendicular BD on AD' and compute the required distance AB 

 by the formula 



AB= ^AD*+BD* 



EXAMPLE. If, in Fig. 5, the distance AD is 206.1 ft. and the 

 distance BD is 35.1 ft., what is the distance from .4 to Bl 



