Bruce in Bangkok wrote:
On Fri, 14 Mar 2008 11:16:58 -0700 (PDT), "
wrote:
I don't know how the OP intends to use the compass but 7 degrees is a
lot of error. More than twice what I'd expect from a well corrected
compass. Keep in mind, with deviation, the error will be between
headings so the angle between your two objects could be 7 degrees off
in this case... Good thing he's got a GPS.

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.

This is only true if the error is constant. For example, you don't have
to be concerned with variation if you're only considering the angle
between two objects. However, if, as is apparently the OP's case, the
compass has deviation that changes with heading, then the computed angle
will be faulty. The reported data implies that the angle could be off
by more than 8 degrees, which makes it hardly worth plotting.

Traditionally, a horizontal sextant sight is taken. This can give a
high degree of accuracy. Until recently, such sights were the primary
technique for positioning buoys.