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    Re: Problem shooting index error
    From: Bill Lionheart
    Date: 2019 Oct 15, 16:28 +0100

    Maybe when you say "a round of three start" perhaps you mean
    implicitly that they are chosen so the azimuths cover greater than 180
    degrees as of course is best. (And I wonder if there is a CN term for
    such a well chosen round?). My point is that if you have ONLY index
    error then the true position is at the incentre, the centre of the
    inscribing circle of the cocked hat.  The point being that the index
    error would be a fixed distance from the lines of position on the
    chart. The inscribed circle is tangent to each line of position and so
    its centre, the incentre, is the same distance, the radius of the
    incircle and your index error expressed in nautical miles,  from each
    line.   However if the situation is not so ideal so that one of the
    index errors puts you outside the cocked hat then you will be at the
    centres of one of the excircles, tangent to all three lines of
    position but outside the cocked hat.
    
    I have a feeling that this discussion has arisen on NavList possibly
    several times before already and I should provide a link. Not actually
    seen it in book though (and I fully expect one of our community to
    provide a link to a very old French book in which it first appears, as
    was the case for the symmedian point and identically distributed
    random errors).  In any case I would appreciate if there is a name for
    these "good and bad rounds of three stars".
    
    
    On Mon, 14 Oct 2019 at 18:48, Ed Popko  wrote:
    >
    > And on a slight tangent should you try stars: Greg pointed out, many posts 
    back, that if your sextant has a slight index error in it (either on or off 
    the arc) and you do a round of stars for a three body fix, the only effect 
    the IE will have is to make the cocked hat bigger or smaller than what it 
    might be if you had no IE at all. In other words, the center of the cocked 
    hat, however you want to analyze the triangle's center (and there are more 
    than one way), is unaffected. The center remains the same
    >
    

       
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