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    Re: Magnetic Variation. (has been: Magnetic Declination)
    From: Doug Royer
    Date: 2004 Feb 13, 10:57 -0800

    Thanks for the well thought out response George.Yes,I see my thinking was
    narrow as I thought only of what I am used to useing or doing.
    However,a few questions/comments are still in order.
    Keiran's statement below still has me interested.I will wait for him to
    reply and explain what he was trying to relate.
    >Keiran stated he has a way to find local variation useing " a
    >sextant,compass and amplitude".
    Your points below are noted.
    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.
    Yes,differant cargoes will effect the compass.So that is why some cargoes
    are stowed for transit in differant sections of the hold or on deck.Or the
    same cargoes are split up and stowed in differant areas.
    The bigger vessels I'm familiar with have 1 magnetic compass,1 main gyro
    compass and many gyro repeaters placed throughout the vessel.
    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.
    The below is correct.
    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.
    Yes sir,the below is true to a point.If the measurement is taken with a hand
    held compass at differant points of the vessel the deviation will
    change.That is why,I believe,we were taught to not take into consideration
    the deviation of the hand held magnetic compass(to a point,as you stated the
    dev. can be large).I've never taken an amplitude or azimuth useing a hand
    held compass while on board.Always done with the either of the wing or
    midship gyro repeater.
    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.
    I always have questions!
    I hope this has answeed Doug's question, but if not, I hope he will ask
    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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