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The latest image shows how a ring of 14 support posts could be erected outside the existing octagon. Spacing on an accurate circle could then be managed with external plywood templates cut to exact size. Though, even here there is ample opportunity to get it all horribly wrong.
Plywood arcs in these huge radii are simply not stiff enough unless made very thick [and heavy.] I was using crossed planks fixed across 3 meter circles for the last build. Now add an extra meter in diameter or half a meter in radius. Without being allowed crossbars to maintain perfect roundness.
It should be obvious that I cannot stretch a line or [much better] a radius board from some imaginary centre within the existing building. Nor use a tape measure for that matter. There is the existing building's plywood cladding and the huge pier posts blocking access. As well as the eight upright, perimeter posts all getting in the way.
So setting out the new post holes accurately is going to be a bit of a nightmare. It was difficult enough on bare gravel when I built the octagon. The bulky concrete anchors in their freshly dug holes simply didn't allow for micro-adjustment. Not helped by the hot and dry conditions which made the gravel looser than normal.
Yet another idea popped into my noddle. I could use a metal rod to measure the radius from the centre point of the new building. This would require only a small hole be drilled through the octagon's cladding. Specific post hole positions can be measured externally using the template arcs and a tape measure. There is absolutely no need to measure out to a particular post at exactly the required angle.
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