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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.
>
>Lisa,
>
>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@unb.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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