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    Re: Fix Maximum Probability Positions
    From: Hewitt Schlereth
    Date: 2013 Mar 24, 11:53 -0700

    Exactly right, Geoffrey.
    My practice was to keep a plain vanilla DR - just course and speed, or 
    distance by log, if the boat had one. Then when I had a fix - either running 
    or two-body, I'd compare fix and DR. 
    If we'd been going to windward, and the fix had us what felt like a reasonable 
    distance to leeward of the DR, okay. To windward of the DR, no way. 
    Close-reaching, on the other hand, If the fix was to windward of the DR, that 
    was within the realm of possibility.
    Even in a "known" current such as the Gulf Stream, I kept a plain DR. Then if 
    the fix-DR distance made sense, okay. If the span said the Stream was running 
    10 knots, nö way. 
    A friend on whose boat we sailed many offshore miles together, called my DR a 
    "pure" DR. When I was aboard I did the celestial and he kept what he called 
    an "estimated position" in his head. Whenever I had a celestial fix (running 
    or two-body), we'd check his against the celestial. Usually we were in the 
    same ballpark. (I need to say my old shipmate knew his boat so well he would 
    usually have the day's run within 10% of what the celestial showed. In 
    effect, his brain contained an internal inertial navigator).
    Sent from my iPad
    On Mar 24, 2013, at 12:05 AM, "Geoffrey Kolbe"  wrote:
    > John Karl wrote:
    >> ----------
    >> Other than Paolo Borchetta (and maybe a couple others, if any)
    >> replies to my posts on the relative merits of the traditional
    >> running fix (TRF) have really grappled with the concept under
    >> scrutiny. Many simply state that the TRF works fine and doesn't need
    >> improvement, but they give no justification for the assumptions
    >> behind the TRF.....
    > In figure 1 of your paper, you show a circular area of uncertainty
    > around your DR, which given no other information is reasonable enough.
    > But in figure 5 you show an elliptical area of uncertainty, for which
    > I can see no justification.
    > Suppose, in figure 5, your course had been along the LOP1 so that DR1
    > was in fact on LOP1. What would your area of uncertainty around DR1
    > look like then? Since your original DR started out as a point (as
    > shown in the figure) it would be circular, just as in figure 1.
    > The point about advancing an LOP along the vector of your course and
    > distance run is that it does not create a new DR position, as you
    > attempt to do. It retains the LOP as an LOP, but makes use of the
    > estimated course and distance run information in a reasonable way to
    > re-locate the LOP.
    > Celestial navigation is not actually a method of navigation.
    > "Celestial navigation" is a misnomer. "Celestial navigation" does not
    > give you course steered, distance run, or your most probable current
    > position, which is what "navigation" is about - having a good idea
    > where you are at any given moment in time. Position fixes using
    > celestial bodies are a way to get an independent check on your DR
    > position. The navigation method used is actually dead reckoning
    > first, last and always.
    > It follows then that polluting your celestial fixes with where you
    > arrogantly think you are is a dangerous way to proceed. It throws out
    > the independent check.
    > In your figure 5, if your LOP2 is good, then that should give you
    > cause for concern that your course steered was not what you thought
    > it was. There may well be a current flowing to the East which you had
    > not suspected. You, however, are not treating CN as a check on your
    > DR, you are using CN in combination with estimated course and
    > distance run to create a DR, which - I respectfully submit - is dangerous.
    > Geoffrey Kolbe
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123050

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