Re: [APML]: Great RA Alignment tool

[Wil Milan <wmilan@airdigital.com>]

Ben Gomes-Casseres wrote:

> Wil, what is the difference between *variation* and *deviation* in this
> context? In Curacao, I set my adjustable compass to point 7 degrees to the
> west of true north, as my marine map shows that as the local magnetic
> "variation." I hope that's right...but what is deviation then?

Magnetic variation is the offset of magnetic north from true north due to
one's location on the Earth. The variation lines are plotted on many maps
(such as aviation and marine charts). Magnetic variation is strictly a
geometric effect due to the magnetic pole being some distance from the
geographic pole.

Magnetic *deviation* is a different thing altogether, and it is due to local
magnetic effects. If an airplane flies near a mountain with a lot of iron ore,
for instance, the nearby iron mass will cause compass readings to deviate some
amount from magnetic north. Magnetic deviation is usually a very small-scale
effect and because it can vary so much over short distances they are generally
not plotted on maps. Navigation charts often contain notices warning of
unusual magnetic effects where such effects are reliably known, but because
they are so localized there are no specific settings given for correction.

Wil M.
--
"When I consider your heavens, the work of your fingers, the moon and the
stars which You have set in place, what is Man that you are mindful of him, or
the son of Man that you care for him?"  -- Psalm 8