A record survey by PhoToPlan

Drainage problems with the pavement on the gun deck of Martello Tower 24  this tower required a full record of the historic paving prior to lifting for English Heritage.

This is a rectified montage of the gun deck pavement and firing step of a Martello tower. It was achieved by 4m PAP (Pole Aerial Photography) in a single day on site. The pavement  level is  a stitch of 9 images, the firing step  12. The images were rectified in PhoToPlan and then balanced and feathered in Photoshop.

Control for the rectification was by REDM to detail points collected in TheoLt:

In this plan I plotted the wall top from intersection in PhoToPlan 3D, orientated to the same control points from our reconaisance KAP imagery:

This tower is impressively armed with a mighty 24pdr; we had to wait until it was lifted before we could record the pavement condition in advance of works to stop the leaking, visit when it rains and you get wet!

Once the pole is set up and the rig hoisted it’s a case of working as quickly as possible to get even illumination across the whole photo block…and taking advantage of rare moments of excitement like the bit where we have to negotiate the flag pole.

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.

This documentation project is a great example of what can be done with PhoToPlan, Photoshop, AutoCAD TheoLt and….

…a camera on a stick!…

So in 4 easy steps we got from ‘no cover’ to ‘ortho’ cover.