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    Re: Round-off
    From: John Karl
    Date: 2009 May 15, 20:17 -0700

    Peter wrote:
    >
    > I think the simple answer is that for practical reasons you have to give
    > consideration to how probable error leading to risk is. � A statistical
    > analysis can do this. �Risk can thus be usefully evaluated, leading to an
    > informed decision as to action, if any.
    >
    > If you considered that the maximum potential risk should always be applied
    > then I don't think you would go sailing at all.
    
    I wasn't talking about sailing risk.  But rather, I was addressing
    round off error in sight reductions.  Certainly in some circumstances
    (such as learning, navigating in the open ocean, or lifeboat
    situations) large uncertainties (like 10 miles or even more) are quite
    acceptable.  But when Cnav is really important why introduce
    unnecessary uncertainties in the sight reduction??  Why add more to
    the inherent uncertainty in the observation??  Using H.O. 229 or a 10-
    digit calculator and maintaining 0.1' arithmetic should produce a
    sight reduction accurate to about 0.5'.  However, remember Almanac
    data is only guaranteed to 0.25 for the sun and 0.3' for the moon.
    And even when double second difference interpolation is required in
    H.O. 229, that table's accuracy can drop to 0.3', further adding to
    the uncertainty.  So in some cases, exclusive of observational error,
    the sight reduction itself can have an error of up to 0.8'.
    
    Even in keeping statistics to evaluate observational skill, one needs
    to minimize the arithmetic error in the sight reduction calculation,
    and needs to know just what that uncertainty is.
    
    JK
    
    
    
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