110 STADIA AND PLANE-TABLE SURVEYING 



a point P,.Fig. 4, not platted on the board, from which three 

 points A , B, and C platted at a, b, and c, respectively, are 

 visible, but whose distances from P cannot conveniently be 

 measured. To plat this point fasten a piece of tracing cloth 

 over the plane-table paper; orient the table approximately 

 with the eye, and select on the tracing cloth a point p' approxi- 

 mately corresponding to the true position of p with regard 

 to a, b, and c, plat the lines p'c', p'b', and p'a' as if p' were 

 the correct point p. Then unfasten the tracing cloth and 



P 



FIG. 4 



shift it to the position p"a", p"b", and p"c", in which each 

 of the lines p'a', p'b', and p'c' pass, respectively, through the 

 points a, b, and c. The point p" is then over the exact posi- 

 tion of p and -can be pricked through with a needle point. 

 The plane table can then be oriented accurately by means of 

 any of the lines pa, pb, or pc. 



The Two-Point Problem. When only two points A and B, 

 Fig. 5, platted at a and b are visible, but inaccessible, the platted 

 position of a third point C may be determined by establishing 

 through it a line parallel to A B and orienting the table by means 

 of that line. The field work is as follows: First, set up the plane 



