The rigidity of the A-frames and elevation axis may not be perfect.
Hence, it may be that the inclinometry measures from the bottom beam of the
A-frame does not describe the rotation of a rigid A-frame, and hence cannot
be used to determine the motion of the apex of the A-frame. It may be
necessary to introduce a multiplicative factor f describing the
flexibility of the A-frame ( f < 1) and to determine
f experimentally, as described below.