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Unfortunately, recent postings have changed this threadname to "Magnetic
Declination", and I have done my best here to change it back to the proper
nautical name of "Magnetic Variation".

Frank Reed wrote, under "Magnetic Declination"-

>Bear in mind that there are SEVERAL competing, similar terms like
>"declination" and "variation" for the difference between true north and
>magnetic north.
>It's always best to say what you mean when you write about this material, and
>there's no point being too pedantic about which one is "right".

In a maritime context, there's only one term that I know of in
English-language use, and that's "variation". Frank is right in that
physics textbooks (and therefore presumably geomagnetic surveyors and
land-mappers) use "magnetic declination" where we would use "magnetic
variation", and that usage also occurs in French mapping (not sure about
French charting).

When he says "Bear in mind that there are SEVERAL competing, similar terms like
>"declination" and "variation" for the difference between true north and
>magnetic north.", I can't think of any others (which does not mean they
>don't exist). Perhaps Frank can remind us. There are other words used for
>the angle between magnetic North and grid North (on a projection such as
>UTM where the grid isn't North-South), abortions such as "grivation", but
>that's a different matter altogether. Mariners can confidently go on using
>"variation" to describe the angle they need to adjust their compass
>courses, but if they come across "magnetic declination", it means exactly
>the same thing.

Doug asked this fair question-

>In my training and experiance one uses amplitudes to find the compass
>error.By adding or subtracting the variation one finds the compass deviation
>at that heading and time.With out knowing the deviation how can one know the
>variation useing amplitudes?In your explaination is the sight taker useing a
>hand held bearing compass or a sight vane on the ship's compass or
>repeater?There could be a differance because if one is useing a hand held
>compass to observe the amplitude one doesn't take deviation into account as
>one does on the gyro or ship's compass's sight vanes.
>Let's tackle amplitudes 1st.I know of 2 ways one can find an amplitude of a
>body.By observing the body's amplitude on the visible or celestial horizon
>with a compass and useing the amplitude tables or by calculateing the body's
>amplitude on the visible or celestial horizon.
>In either precedure above the resulting amplitude is compared to the
>observed amplitude thus finding the compass error.
>Keiran stated he has a way to find local variation useing " a
>sextant,compass and amplitude".How can this be done?Am I missing something
>or are my thoughts to confined ie. ship board application only?

=======================

Doug is in a special situation (though a very common one), in needing to
know his compass errors on board a steel ship.

That was why I said, in my original posting-

"If there's any iron around, then the needle is also affected by the local
deviation, which varies with the course of the vessel. But in the absence
of any such local deflection (or if it's been well-compensated out), the
variation is what remains  "

Doug needs to know his total compass error, which is the sum of the
magnetic variation (which varies from place to place, and from time to
time, but isn't affected by his ship), and the magnetic deviation (which is
the error in the compass caused by the unavoidable magnetism of all the
steel in his ship, and varies with her course).

In an engineless wooden vessel, such as were used for early magnetic
measurements, there was no, or negligible, deviation, unless some idiot had
mounted a cannon near the poop, or someone was standing at the binnacle
with his sheath-knife in his belt. The oceanographic recearch yacht
"Carnegie", in the 1920s, was a wooden vessel which even had a cast-bronze
non-magnetic gasoline engine (and which ended her days with an explosion
when filling with fuel). Many of us who sail wooden or fibreglass
small-craft, with a small engine well spaced from the compass position,
find such small compass deviations that they are not worth worrying about;
though that doesn't apply to all such installations.

In such a vessel, measuring the total compass error is equivalent to
measuring the local magnetic variation, if the deviation can be neglected.

The same applies to Kieran Kelly, travelling overland in Australia with a
(non-magnetic) camel, though he needs to look out for the knife, gun or
billy-can that he keeps in his tucker-bag down by the billabong.

Doug, in his steel ship, is in a much worse position. His compass would be
dominated by the local magnetism of the steel that surronds it, if those
effects had not been carefully compensated out. That was one of the
strongest arguments against the introduction of iron ships in the 1800s,
and many vessels were lost because of immense compass errors, before the
reasons, and the compensation techniques, were understood.

Doug can measure his total compass error, from a Sun amplitude, as he
describes, but that's the sum of variation and deviation. How does he
disentangle them?

Well, why does he want to know? Because at some time, a compass-adjuster
has done his job. What he has done is to install an assembly of magnets and
iron balls that are designed to null the effect of the ship's magnetism, on
all courses, at the place where the ship's compass sits. Note, in passing,
that he is not correcting the compass itself, which for this purpose we can
take as error-free: he is correcting the space that the compass sits in,
for the effect of the ship. You could put another compass into that same
spot and it would be corrected just as well.

But compass adjustment is not an exact science. The adjuster does his job
as well as he can, but he has access to the ship only at one spot on the
Earth's surface. As she travels, the Earth's magnetic field changes, very
significantly. It's only the horizontal component that's useful in aligning
the compass. Near the equator, the field is roughly horizontal, but at
higher latitudes, it's angling upwards or downwards. Where I live, it
points down at about 67 deg from the horizontal. The adjuster does what he
can to compensate for such changes in the magnetic dip, but it's always an
approximation.

And the deviation can change, for other reasons than a change of dip. A
lightning-strike can have an instant effect. Some cargoes can disrupt the
deviation: the most notorious being railway-lines. What effect does a full
load of containers have on the compass, I wonder; or a loading of tanks, on
a landing-craft? Some idiot may have placed a loudspeaker near the master
compass. The initial magnetisation of a new vessel can diminish in its
first months.

When a navigator today measures total compass error, variation plus
deviation, from (say) a Sun amplitude, he usually has a good local value
for variation from his chart. What he is trying to obtain is a good figure
for the deviation of his compass, on a particular course, to compare with
his deviation card or to confirm the accuracy of compass correction. If he
makes a habit of doing this, on various courses, it will (or perhaps won't)
give him confidence in the accuracy of his compass courses, or warn him
it's time to call in a compass adjuster, or perhaps find that bit of steel
that some fool has left in the binnacle.

Doug asks whether the compass error should be measured with a hand
bearing-compass or with the ship's standard compass. On his steel ship, my
guess is that there's nowhere on board that's immune from the magnetic
distortions that its steel causes, so there's nowhere to take a hand
bearing-compass where it would give sensible readings. In his case the only
useful comparison is of the standard compass with astronomical azimuths, or
with well-known transits of landmarks on his chart.

Those of us with small fibreglass craft have it easier. If you take a hand
bearing-compass well away from the engine, say toward the bow (but away
from the anchor) then you can rely on it showing an accurate magnetic
bearing to some distant landmark. Compare that with a similar bearing given
by sighting over the steering-compass, repeat for a number of different
courses of the vessel, and you have a deviation card for the
steering-compass.

I hope this has answeed Doug's question, but if not, I hope he will ask further.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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