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    Re: Position lines, crossing.
    From: Bill Noyce
    Date: 2006 Dec 10, 19:40 -0500

    I agree with George that a vessel's true position is outside the triangle
    formed by three observations 75% of the time.  But we need to be careful
    to distinguish two things that are aften treated as "the same".  One is
    the actual position (as might be shown on a perfect GPS, for example).
    The other is the FIX that the navigator chooses to put on the chart,
    enter in the log, etc.  We often do our navigation with the assumption
    that a FIX tells us where we "really are", and that's one aspect of the
    dangerous assumption George is warning about.  In fact, a FIX should
    be drawn -- or considered -- as a smudge, whose size depends on how
    confident we are about whatever info it was derived from.  I think that's
    pretty well understood here.
    
    It's a common (and reasonable) practice to draw the FIX in the center
    of the triangle.  That's fine, as long as we remember that it's really a
    smudge.  And, despite the fact that we draw the FIX inside the triangle,
    there's nothing that says our ACTUAL POSITION is inside the triangle.
    Most likely it is not!  That's all George is saying.
    
    How often does it matter?  In the middle of a big ocean, probably never.
    But if you're near the Scilly Shoals, or something, it could matter a
    great deal.  "I know I'm safe, because no part of the triangle leads
    to running aground" -- then "crunch!"
    
    So by all means, use that FIX as a starting point for your DR plot, but
    remember that the starting point is a smudge, and the waypoints you
    plot are bigger smudges.
    
    Hoping I've added light, without heat...
        -- Bill
    
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