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    Re: Fw: Re: Still on LOP's
    From: Brian Whatcott
    Date: 2002 Apr 27, 12:25 -0500

    At 04:37 PM 4/27/02, George wrote:
    >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....
    >George Huxtable.
    Everybody is now sounding a note of exceptional unison.
    It must be time for me to interject!
    As we are talking of allowing appropriate offsets from a cocked hat, here
      is a rough and ready suggestion where nothing better is available.
    Measure the length of the longest side of the cocked hat.
    Use that as radius from the MPP in the cocked hat's interior  to draw a circle
    for a circular estimate of position.
    I suggest that has a reasonable chance of  in fact covering the
    actual position. I think it puts the actual position in the circle, more often
    than not.
    Brian Whatcott
       Altus OK                      Eureka!

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