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re accuracy human eye/ reading scales.

Gary/ Hanno.

This is interesting. I'm no expert, but I remember being told years ago that many people have better "Vernia acuity" (like hyperacuity) allowing them to distinquish positions between scale lines of down to 0.5' whereas the normal 20/20 vision limit for alphabet letters is about 1', limited by the retinal cone structure. The increased Vernia acuity is  apparently helped by higher brain cerebral and psychological functions. Practice and experience can improve it too, and stress etc make it worse.

I think that expalins why I can consistently get better than 2' with Bygrave (even at the "bad" end of the scales)  and even 5' with the BN. Much better than theoretically possible.

Probably depends on the person. I make more mistakes reading log tables and even the Doniol, but that is me. I also press the wrong buttons on calculators which results in wrong answers to 10 decimal places!

Chichester says this too and preferred the Bygrave to 4 figure logs. Famous RAF Pathfinder leader Don Bennett likewise and he had Air Tables by then.

In really stressful real world scenarios, I would always check with the GPS who doesn't seem to worry about anything!

I still hope Gary will take up the challenge of an aerial, real world comparative trial of Bygrave SR v Bygrave flat v BN, v Hv Doniol!

Best wishes

Francis