Figure demomstrating inaccuracy involved in using the mid - contour 



(O') as the turning point rather then the correct turning point 



0, such that OA = OB. The distance OC is given by the expression, 



tan 



(a- ^)- ton (oc- Aoc) 



tan oc — tan ( cc — A oc ^ 

 where Ay is the distance between the contours d, and dg- 



Ay 



FIGURE- 3 



30 



