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    Re: Fix Maximum Probability Positions
    From: Hewitt Schlereth
    Date: 2013 Mar 24, 17:58 -0700

    John, what I mean by a DR is the boat's position based on the course steered 
    and distance run since the last fix. Just those two things. 
    
    My experience over the years was that over a period of four or five hours 
    courses were steered pretty accurately, especially on racing boats, where the 
    helm changed every half hour. One person steered a little high; the next a 
    bit low, etc., and resulted in the course aimed for.
    
    The biggest variable I found was distance run. But it didn't take too long to 
    figure out the speedo or log was reading some percent high or low, once I had 
    several two-body fixes.
    
    I stuck with what my friend called a "pure" DR because it was based on 
    measurements: 1) by the speedo or log for distance;  2) by my observing the 
    telltale compass as the helm went from hand to hand. Because this way of 
    keeping a DR rests on observable data, I always felt it reduced the DR 
    uncertainties as far as they could be. As time passed and fixes accumulated, 
    it became increasingly clear how closely the distances run and courses 
    steered approached the truth. In other words, this method required the least 
    amount of estimation on my part.
    
    Hewitt
    
    
    Sent from my iPad
    
    On Mar 24, 2013, at 3:42 PM, "John Karl"  wrote:
    
    > Hewitt,
    > 
    > Sounds like a semantics question here.  By DR position I mean the 
    navigator's best estimate of position when the second LOP was required.  In 
    all my figures the DR means the ship's current position from knowledge of 
    factors affecting the ships track and distance made good from the ship's last 
    upgraded position.
    > 
    > Maybe we are using different definitions of DR position.  There are many 
    ways of plotting running fixes, but I tried to use the simplest for 
    discussion.
    > 
    > Does this make sense??
    > John K
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