Originally posted by mtthw
'Compass' is acceptable according to the OED, although is says compasses is more common. 🙂

I am common!

Chambers only lists compasses (or pair of compasses) but I stand corrected.

Originally posted by mtthw
You sure about that? Right-angled to approximately level will only give you approximately the highest point.

Anyway, better hope that the pencil doesn't have an eraser on the end. Or been chewed. All seems very approximate to me.

You sure about that? Right-angled to approximately level will only give you approximately the highest point.

But would my error be more than the thickness of a pencil and the human error inherent in the positioning of the needle?

The pencils that come with compasses don't have erasers. They're cut flat on the end...at least the cheap ones I bought at the grocery store were.

You cannot avoid error.