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    Re: Compasses
    From: Martin Gardner
    Date: 2002 May 30, 07:15 -0700

    Thanks for the on list and private replies to my query on fluxgates.  You
    expressed proper skepticism about a deviation card which showed no errors,
    and a suspicion of the compressor.
    George asked:
    > I wonder whether Martin was able to take some magnetic bearings on
    > landmarks or perhaps the Sun on that day, in order to establish which
    > compass was showing the error. Even a true course-over-ground, from GPS,
    > would suffice to show up a 10 degree discrepancy if allowance could be made
    > for any currents.
    Nope, sorry.  When I'm next on this boat, I'll try to check.  We actually
    had 6 gps receivers aboard (with only five crew).  Maybe they ganged up on
    the fluxgate....
    Then he continued...
    > We need to appreciate what a fluxgate compass is being used for. Sometimes,
    > it's just there to provide some direction feedback for a self-steering
    > system, in which case even quite gross compass errors may be acceptable.
    > But if it's to provide a bearing reference for navigation, or a North-up
    > heading reference for a radar display, deviation errors need to be well
    > corrected, and variation needs to be well known.
    Very astute: I noticed the error when I saw that the helmsman was off course
    using the autohelm.  I never got the exact error, as the autohelm displays
    desired course, not actual course, and it didn't occur to me to power up the
    radar to see exactly what the fluxgate thought our heading was.  So my 10
    degrees is a guess.  Next time I can try some experiments, like turning on
    the compressor and the radar.
    Thanks again all,

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