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    Re: Fix Maximum Probability Positions
    From: Hewitt Schlereth
    Date: 2013 Mar 26, 14:45 -0700

    John-
    
    It's always seemed to me that data based on the onboard compass and log are 
    better than data from tide tables and pilot charts. After all, tide tables 
    give the time the tide changes in a computer algorithm, the pilot charts show 
    the long-term averages for winds and currents. Neither can tell you what is 
    actually going on around you at your present place and time.
    
    Distance run and course steered, on the other hand, are based on direct 
    reading of on-board instruments at your present time and place.
    
    If you keep a DR based solely on compass and log, the distance between your DR 
    and LOP has the net of all the instrument and personal errors and all the 
    effects of wind and sea. When DR and LOP are in front of you, THEN is the 
    time to use any additional information and make a judgement as to whether or 
    not the position makes sense. 
    
    Hewitt
    
    
    Sent from my iPad
    
    On Mar 26, 2013, at 1:50 PM, "John Karl"  wrote:
    
    > Hewitt,
    > 
    > Yes in the estimation approach, one uses all the info available PRIOR to 
    acquiring the new, and more accurate, LOP.  There's nothing wrong with 
    ignoring currents and drift if there's no info on them, or if you simply 
    chose to.  Then the location of the EPRF along the new LOP has that 
    uncertainty built in, as well as the normal uncertainty.  But that EPRF is 
    still the best Rfix with the data used.
    > 
    > And yes, the distance between the EPRF and the TP (DR, etc.)  is a measure 
    of the component of error in the direction perpendicular to the LOP.  And 
    there's no info on error along the LOP.
    > 
    > But the traditional running fix gives no error information.
    > (The error is all in its assumptions.)
    > 
    > JK
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