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    Re: Magnetic Variation. (has been: Magnetic Declination)
    From: George Huxtable
    Date: 2004 Feb 13, 16:04 +0000

    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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