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    Re: Magnetic Variation
    From: Scott Owen
    Date: 2010 Mar 01, 19:07 -0600

    Guy Schwartz wrote:
    > Hello all:
    >
    > I just took a test today and one of the questions had to do with magnetic
    > variation. The answer to the question was that magnetic variation changed
    > seasonally. I checked Chapman's, Dutton's and Bowditch, no mention of
    > seasonally adjusting?
    >
    > Anyone have any information or was the test wrong?
    >
    > Thanks
    >
    > Guy
    
    I suppose that "seasonally" could be interpreted to mean that magvar
    does indeed change and that this change is incorporated for navigational
    purposes on an "annual" basis, though "seasonally" in most people's
    lexicon means quarterly.  Today, in order to get the correct local
    magnetic variation one consults the compass rose on a modern chart and
    with some rather simple math the job is done.
    
    As Andres points out almost everyone uses the World Magnetic Model for
    determining magvar.  At the bottom of the page from the below link are
    two pdf files one showing 2010 magvar and another "secular rate of
    change" whatever that is.
    
    http://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml
    
    From a historical perspective, I am sure at some point, cartographers
    and navigators accounted for magvar without the aide of electronic
    computers, charts with magvar data in the compass rose, and worldwide
    airborne magnetic surveys.  So if there was an "old school" computation
    for finding local magvar, what is it?
    
    Scott
    
    
    
    

       
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