For completeness the pavement survey was required to cover the firing step so the images are rectified again but this time to the firing step plane. The control points are snapped onto the joint lines from the TST survey by TheoLt. 12 images got me around the ring.
I had hoped there would be a deep enough chord of the circle covered in each shot to make use of the minimum number of points I measured to describe the jointing of the circumference rail support blocks, this turned out to be the case but more points would have been better, some of the projected images came out a bit stretched!
Next the central pivot column is rectified so that the final montage is presented to a consistent scale:
The pivot post is easy: 6 points and its done! Bringing the pieces together completes the cover: The 3 planes are combined into one single orthographic projection, there was a great deal of fitting needed to get the coverage of the pavement right up to the base of the central pivot post from oblique imagery taken with the pole on the firing step.
Re-inserting the montage back into AutoCAD and checking by PhoToPlan rectification showed a 9mm average discrepancy between the montage and the control points.
So in 4 easy steps we got from ‘no cover’ to ‘ortho’ cover: