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    Re: Annual Increase of Variation
    From: Steven Wepster
    Date: 2002 Feb 12, 14:11 +0100

    >What is the cause of the "annual" portion of the magnetic variation on
    >the compass rose?
    The earth's magnetic field is not constant but subject to ... well,
    variation. The annual correction is a local linear approximation to the
    change. As such it is not valid for ever; its value should be revised after
    some time on the basis of regular determinations of the variation.
    If the annual change is given as 1' -- 3' then after 20 to 60 years the
    variation has changed 1 degree. I don't know the policy of the hydrographic
    office: whether they update the compass rose only at new editions of a
    chart or also at reprints. If the former applies, and no new edition has
    occured in 20 years, then you don't see a change in variation on the chart.
    Steven Wepster                          wepster@math.uu.nl
                                            tel +31 30 253 1186
    Mathematisch Instituut
    Universiteit Utrecht
    PO Box 80.010
    3508 TA Utrecht
    The Netherlands

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