Layout Pt.3 Wrong!

COMPLETELY WRONG! Ignore everything I have written so far.  Never assume without checking!

I was making the simple assumption that a 3m diameter [circum]circle could be simply divided by 8 for the number of sides to calculate their lengths. I had been using side lengths of 1/8 of the circumference of the 3m dia. circle = 118cm. In fact I should have used 114.8cm side lengths between post centers. 3.2cm difference doesn't sound like much but quickly adds up as one works around the octagon. By the 5th post the error is already 5 x 3.2 = 16cm. = 6.3" error. While 8 x 3.2= 25.6cm = 10"!

I couldn't for the life of me see how my carefully measured layout could be so wrong. A straight edge across the center between opposite posts [1&5] missed the center post by several inches! [The so-called long diagonal] I even used a laser distance measuring device [rangefinder] to lay out my  perpendicular to the shed wall. Then carefully marked the wall 3m apart at 1.5m centers from the perpendicular to within 1mm. I stretched my string line taut to perfection and my post footing layout was still all over the place!

The posts do in fact lie on the same circle which joins the 8 points of the octagon. But, to measure their distance apart using 118cm I would have needed a curved rule of 1.5m radius. Grrr! 😳

Geometry: Regular Octagon Calculator   Enter any one dimension.

Oh, for the perfection of hindsight before the event! 😁

After considerable further excavation and the endless struggle to remove two more large boulders I discovered my new post spacing was now too little. By the time I reached post 5 it was lagging well behind a line between 1 & 5. Which should have crossed the diameter. I had used 114cm per side this time but it seems I must use an intermediate number and hope for the best. 116cm per side at 1.5m radius should hit the target. Famous last words!

But it did work and I now have only to bury the last three concrete footings and back-fill after checking the final dimensions. It is almost a shock to see the footings aligned on opposite sides of the octagon. Why couldn't I do that the first time?

Thursday: Another exhausting morning digging up huge boulders to sink the footings to the correct level. It looks like a battlefield and it really was! The boulders were all buried fast in the soil under the 2' of overlaid sand and gravel. Some footings still need to be lowered slightly but they are, at least, properly spaced at last. There are various water pipes for levers and split logs to slowly lift boulders up to the surface so they can be rolled away. [With great difficulty!] The largest boulders probably took 15 minutes of working to breathless exhaustion, at intervals with rests in between, to move off the site with a crowbar. I used to move rocks several feet in diameter using just a length of narrow gauge railway line as a lever. But that was thirty years ago. I'd probably struggle to lift the rail these days.

Click on any image for an enlargement.

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