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    Re: digital compass, etc
    From: Lu Abel
    Date: 2000 Sep 11, 11:25 PM

    Let's step back to the original 360 or 720 degree turns.  The
    microprocessors in electronic compasses can record the compass reading
    fractions of a second apart.  The compass software assumes you've made a
    smooth turn (ie, changed your boat's direction at a uniform rate of speed),
    so any deviation from that in the compass readings it records is, well,
    deviation.
    
    It sounds as though your Furuno goes one step further and tries to figure
    out if the compass's lubber line is aligned with the boat's keel by having
    you take a bearing on a fixed object.  By having you input the bearing, it
    can correct the compass reading for a misaligned lubber line.
    
    While many courses and texts give methods for developing a deviation table
    for a convention compass, they rarely mention that the resulting table can
    be used to detect a misaligned lubber line.  If one has determined
    deviation by taking compass bearings on one or more distant objects, the
    sum of the westerly deviations should exactly match the sum of the
    easterlys.  If not, the compass's lubber line is misaligned.  A way to see
    this visually is by plotting the deviation table on a piece of graph paper.
     Draw a horizontal axis across the center of the paper, divide it into 0 to
    30 degrees.  Then plot  westerly deviations as positive values and
    easterlys as negative.  When the points are joined together result should
    look like a sine wave centered around the horizontal axis.  A lubber line
    misalignment effectively shifts the horizontal axis upwards or downwards,
    making more of the sine wave be on one side of it than the other.
    
    Why am I so hot on lubber line misalignment?  Because I bought a brand new,
    high-quality 36' sailboat some years back.  Immediately made a deviation
    table for its compass which I checked periodically.  Wasn't till several
    years later that I heard about summing the deviations and, surprise, my
    lubber line was off by 2 degrees!  That's a big error in all but he
    shortest courses, so I've become a misionary for doing this oft-ignored check.
    
    Lu Abel
    
    
    At 11:51 AM 9/11/00 -0400, you wrote:
    >I've got a Furuno electronic compass on my boat (connected to the radar).
    >This is swung by turning the boat slowly thru 360 degrees.  I also have a
    >autohelm 3000 autopilot this is swung in a similar manner.
    >Except that in the autohelm it is necessary to then point the boat on a
    >fixed bearing and remove any offset in the deviation.
    >This is not necessary with the Furuno compass. How can the Furuno "know"
    >any offset ?
    >
    >Aubrey.
    >
    >At 08:39 AM 07-09-00 -0700, Patrick McVey wrote:
    >>I don't mind though slightly off subject. You're doing what's called,
    >>"swinging the compass." The sensor (fluxgate I believe) and microprocessor
    >>are testing the sensitivity of all 360 degrees and making a correction
    >>table for each point of the compass (probably all 360 points). The
    >>sensitivity varies due to large pieces of
    >
    

       
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