Question

The ‘fix’ of a plane table with three known points is good if the instrument station lies:-
1. in the great triangle
2. outside the great triangle
3. outside the great triangle but within the great circle
4. on the circumference of the circumscribing circle

Answer 1

Correct Answer - Option 1 : in the great triangle

Concepts:

The accuracy with which a plane table station can be located through three-point problem is known as its fix. The accuracy of the fix depends on the relative positions of the plotted points and that of the location of the plane table station.

According to Lehman’s rule, the ‘fix’ of a plane table with three known points is good if the instrument station lies inside the great triangle and particularly when it is at the ortho-centre and the middle station is much nearer than the other.  

The fix is bad if the plane table station lies on the circumference of the circumscribing circle and three point problem becomes indeterminate and unstable.