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    Re: Position lines.
    From: George Huxtable
    Date: 2004 Feb 19, 00:48 +0000

    Jim Thompson said-
    >5. Bearing of the Body (azimuth). One can only determine the azimuth for the
    >AP, not for the vessel's real position; so the navigator needs to be precise
    >about the time of the sextant altitude, and reasonably confidents in the AP.
    >A celestial EP will only be as good as the AP. The more one has to guess at
    >the AP, then the less confident one can be about estimating where the vessel
    >is on the celestial LOP.
    Response from George
    I doubt that Jim needs to worry overmuch about how close his AP is to his
    true position.
    If he finds that his cestial "fix" is way out (hundreds of miles) from his
    AP, than all he has to do is to choose the position of that "fix" as the
    new AP and do it all over again. In practice, however, it's seldom a
    problem, unless a navigator has been at sea for a long time under overcast
    skies, and his DR is very unreliable.
    And Gary Harkins said-
    >3 CLOP's will have a common intersection at only one place on the face
    >of the earth.  You can plot these using the GP's of the bodies as the center
    >of the COP and the co-altitude as the radius of the COP.  An example of this
    >is a problem used in one of the books I have.  It says that you are somewhere
    >on earth and you take these 3 sights and gives the data (date, time, Hs
    >etc....).  It then asks "where are you?"  At first glance many people
    >think that this
    >is an impossible problem.  However, the plots are possible, although a little
    >cumbersome, because of the typical radius' involved.
    What Gary hasn't explained, however, is how you make those plots of three
    intersecting circles on a flat chart, presumably Mercator. Only over a
    small patch of ocean will a circle map as a circle on a Mercator chart. How
    far away from the intersection were those three GPs, in that example?
    If he was navigating using a globe and dividers, as some Elizabethan
    mariners actually did, then what Gary proposes would become possible in
    real life.
    By defining an Assumed Position, it's possible to reduce the problem to a
    plot over a sufficiently small patch of ocean that the position circles
    become straight lines.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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