Each plywood spot board is secured to 3 stakes driven into the ground with the tops of these levelled and then the spot board screwed to the stakes. No contact by boots with the spot boards is permitted. I first saw Henry use this method back in 1996 when building the Leigh Woods cruck frame - see :-Leigh Woods Cruck Frame
If you look carefully at the Harcourt photo shots you will see string axes running from apex to centre sill and top of wall plate to wall plate. No transit or laser was harmed in the laying out of all 8 reference boards - just string and measuring tape [KISS].
Next post will be about the footings for the frame which will be staddle stones.
Executive Trustee - The Oxfordshire Woodland Group