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    Re: Cannot dispense with the assumed position at sea
    From: Bill Noyce
    Date: 2004 Feb 20, 08:57 -0500

    My attitude is that an AP is simply a convenient place from which
    to compute an Hc, and plot an intercept.  If using certain tables,
    there are constraints on how it's chosen; if you're plotting on an
    actual chart there may be other constraints you'd like to follow
    as well.
    
    But I disagree with the following, if I understand it correctly:
    
    > 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.
    
    One interpretation is that you reduce a single sight, draw a LOP,
    and then drop a perpendicular to it from some previously-chosen
    position (perhaps called your AP?).  In that case, I agree that,
    while the LOP might be pretty good, your position along it has all
    the uncertainty of the previous position.  On the other hand, if
    your observations and sight reductions are good, then your true
    position is much more likely to be somewhere near the LOP than far
    from it.
    
    Another interpretation would be that a round of sights leading to
    a celestial FIX is "only as good as" the AP(s) chosen for reducing
    them.  This I strongly disagree with.  If your celestial fix comes
    out very far from your AP, then use the new fix (or positions near
    it) as a new AP, and do the calculations over again, getting new
    Hc's, Z's, and intercepts.  Really this is just correcting for the
    fact that we use straight lines to approximate circles of position
    -- if we plotted the circles, or otherwise corrected our LOP's to
    better approximate circles, this wouldn't be needed.  But simply
    doing the whole thing over is "simpler" because it uses familiar
    operations, and also serves as a useful check.
    
    The only place this can fall down is if you make just two
    observations, and at least one of them is very high, or the
    azimuths are nearly the same or nearly 180 degrees apart.  Then
    the two circles of position may intersect in two points on the
    globe that are fairly near each other.  A badly-chosen AP can
    lead you to the wrong one of those intersections.  A third
    observation at a quite different azimuth will resolve the
    ambiguity.
    
            -- Bill
    
    
    

       
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