On Mar 14, 4:41 pm, 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.

I don't understand what you are saying. Bearing compasses are almost
always used to get azimuths which are then often plotted on a chart.
They have to read properly to work. If you just want to know the
angle between two objects you don't need a compass at all. For small
angles you can get within a few degrees just siting over you
outstretched hand (each knuckle is about 3 degrees). A cross staff,
peloris, kamal, sextant etc can be used for larger angles. If the
magnetic heading is irrelevant why have a magnet compass at all?

-- Tom.