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    Re: Manual calculation of compass variation
    From: Richard B. Langley
    Date: 2004 Oct 16, 09:28 -0300

    For those who might be interested in a simple introduction to geomagnetism and
    its relevance to navigation (including GPS), see "Getting Your Bearings: The
    Magnetic Compass and GPS"
    . The intention
    of the article was to introduce geomagentism and its use in navigation to GPS
    industry types and was not intended for expert navigators -- such as we have
    on the list.
    -- Richard Langley
    On Fri, 15 Oct 2004, Herbert Prinz wrote:
    >Lisa Fiene wrote:
    >> Can someone explain to me how James Cook did the following:
    >> "Found the Variation of the Compass by the Even'g and Morning Amplitudes
    >> and by 2 Azths to be West 20? 59'"
    >> I've just been reading through some of his log entries, and don't really
    >> understand this one.
    >> Now we know by the compass roses on our charts what the relevant compass
    >> variation is, so have never really thought of how this is actually
    >> calculated.
    >There are two issues here that must be distinguished.
    >1. What Cook did.
    >You find compass variation empirically by establishing true north and
    >comparing it with magnetic north (as per your compass, accounting for
    >deviation). The difference between the two is variation. One common method
    >was to measure the azimuth of the sun at rising or setting with a pelorus.
    >The difference between this azimuth and the prime vertical (i.e. azimuth =
    >90) is called the amplitude. Tables of amplitudes were commonly provided in
    >nautical handbooks. You find an example in Bowditch 1981, Table 28. Today
    >you would compute the azimuth of the sun at rising just like any other
    >azimuth for the intercept method. If you use HO 229 it's a little tricky,
    >because they are designed to find z from LHA, not from Hc. So you have to
    >use them in reverse.
    >2. What the government does when printing compass roses.
    >After having obtained many individual observations such as Cook's, in many
    >places all over the world and over a long time (because variation is
    >changing with time), scientists made a mathematical model of the geomagnetic
    >field (using spherical harmonics) which allows us to predict variation for a
    >given lat, lon and year. This process is not unlike tide prediction, where
    >observations are made in certain key locations and by analysing the
    >oscillations one can interpolate for places in-between and extrapolate for
    >times in the future.
    >The geomagnetic model is not manageable without electronics and its
    >discussion is therefore outside the realm of this list. It is implementedin
    >the firmware of every GPS receiver. Programs for pocket calculators and PC
    >are available.
    >Herbert Prinz
     Richard B. Langley                            E-mail: lang---.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/

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