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    Re: Fw: Re: Still on LOP's
    From: George Huxtable
    Date: 2002 Apr 27, 22:37 +0100

    Dan Hogan wrote-
    
    >How about comparing it with your Dead-Reckoning position?
    >
    > 25 Apr 2002, at 23:31, George Huxtable wrote:
    >
    >[Snip]
    >> This was the attitude taken by old-school navigators in the erroneous
    >> belief that the true position must always be embraced by the cocked hat.
    >>
    >> It interests me what degree of safety Barrie thinks he is achieving when he
    >> takes that part of the cocked hat that is closest to land as his position.
    >>
    >> It has, I hope, been generally accepted by most listmembers by now that
    >> there is only a one-in-four chance that the true position will be contained
    >> within the cocked hat at all. If a navigator insists on assuming, against the
    >> evidence, that he must be within the cocked hat, then he will get a marginal
    >> improvement in his safety by presuming that he as at that part of the
    >>triangle
    >> which is nearest the danger, rather than at its centre. But he is being
    >>unduly
    >> complacent, and there is a strong chance that he may be significantly nearer
    >> still to the danger. The only safe thing to do is to assess the likely errors
    >> involved, in the light of his previous experience, and allow an appropriate
    >> offset. This offset should always be greater, and might be many times
    >>greater,
    >> than the dimensions of a particular cocked hat.
    >
    >Cheers
    >-Dan-
    
    ==================
    
    Dan is quite right, of course. The position from a 3-bearing or
    3-Sumner-line fix has to be given appropriate weight, and combined with
    other information, such as the DR position taken from an earlier fix, and
    perhaps soundings too, where these are relevant and weighted accordingly.
    Everything of any relevance should be taken into account.
    
    The master of a barque which had been beating against the Westerly winds
    and the current swirls off the Horn for a week without a view of the sky or
    the land (and that wasn't uncommon) would put little reliance on his DR.
    
    The true art of the traditional navigator is in estimating what weights to
    attach to the various bits of information, and thinking out how to combine
    them (in his head, most likely) to estimate a position and to estimate the
    possible errors in that position. That's an art which is becoming lost
    since satellites took over.
    
    Comparing a fix with your dead-reckoning position, as Dan Hogan suggests,
    can (if they agree) enhance confidence in the result. That confidence may
    be spurious if, as is possible, both are in error. Only if a previous fix
    was precise and recent, and the sailing in-between has been in an area
    where currents are small and well-known, is the dead-reckoning likely to
    add much to the information obtained in a 3-point fix. On the other hand,
    if the two positions being compared are discrepant, outside the limits of
    the expected errors, then that will at least inform the navigator that
    "something is up" and he will learn to distrust both the DR position and
    the fix until he has discovered what the problem is, or re-measured.
    
    It's all a matter of commonsense, really.
    
    I should add that others on this list (including Dan Hogan) have a much
    wider practical experience in such navigation than I do, so my
    pontifications should, accordingly, be given an appropriate weight.
    
    George Huxtable.
    
    ------------------------------
    
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    
    
    

       
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