On Sat, 15 Mar 2008 09:41:57 +0700, Bruce in Bangkok
wrote:

I had assumed that he was using the glasses to measure the angle
between two known objects as is normal with a hand bearing compass. In
which case whether or not the glasses were reading the correct
magnetic heading is irrelevant.
\
GPS is here to stay, but the sextant has other uses. Measuring the
vertical angle subtended by something, a lighthouse perhaps, of known
size, will give the range.

Casady