PLANE TABLE SURVEYING. 
103 
towards a, turn the table about until the point A is intersected; then, 
clamping the table in this position, turn the ruler and intersect the 
point C, with the edge of the ruler still against the pin at b, and draw 
the line b m :—Now remove the pin to the point a , and unclamp the 
table, place the ruler against the pin at a, and the point b } and turn 
about the table until the point B is intersected ( vide 2); clamp the 
table again, and, haying intersected the point C as before, draw the 
line a n. Through the intersection p of the lines a n and b m, draw 
the line e p q passing through the point c, and, placing the edge of 
the ruler against this line, unclamp the table once more, and turn it 
about until the point C is intersected (vide 3); now clamp the table, 
and it will be oriented, and the unknown point x will be situated on 
the line c p q; to find this point it is merely necessary to place the pin 
at a, and intersect the point A; draw the line A a x. The accuracy 
of the operation is tested by intersecting the other point B in the 
same manner, and drawing the line B b x, which should intersect 
the line A ax on the line c p q, thus giving the position of x on this 
line. 
When the point c, with regard to the point x, is situated on the other 
side of the line A B or below it, the lines a n and b m will intersect on 
the opposite side of the line a b, to that on which c is, and, if the point x be 
situated within the triangle ABC, these lines (a n and bm) will diverge 
instead of converge, in which case they must be prolonged in the opposite 
direction until they intersect for the point p. The accuracy of this result 
depends upon the length of the line c p. 
Third Method. —Fasten a piece of tracing paper over the survey with 
drawing-pins, stick a pin in at any point x on the table (Fig. 5, p. 104), 
place the fiducial edge of the ruler against it and point the sights in turn 
on the stations ABC, on the ground, represented by a b c on the plan, 
drawing lines towards you on eaclr occasion until they meet at x. Now 
take out the pins that fasten the tracing paper to the board, and shift 
it about until each of the lines passes through its corresponding station, 
as shown on Fig. 5. Prick through x, which will be your position on 
the plan. 
In using this method, however, care must be taken to select objects 
placed so that the centre one shall be the nearer, or the position found 
may be considerably in error. 
