Thus we consider here small displacements on the surface of the earth.
Consider a point r on the surface of the earth.
Consider two points r_1 and r_2, which lie on the surface of the earth, near r.
So define...
As the 
's are small we have:-
Thus from the equation's of the spheroid we have :-
Similarly for the second displacement we have...
The cos of u, the angle between these two displacements can be calculated from the
scalar product...
The length of the displacements are :-...
