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    Re: Why Not To Teach Running Fixes
    From: George Huxtable
    Date: 2009 Dec 16, 11:13 -0000

    John Karl, in [11196] writes (again)
    
    "I'm a bit surprised that no one can agree (or can see) that the standard
    running fix uses only the perpendicular component of the advance of LOP1,
    while completely ignoring the component of the estimated track parallel to
    LOP1."
    
    But John, we can all see this. Indeed, it's so obvious, it goes without
    saying.
    
    As Lu wrote, in [11157]-
    
    "Technically you're right that only the component of the course
    perpendicular to the first LOP is the only part of value (or, more correctly
    in keeping with your words, that the component perpendicular is worthless).
    
    And ... so what??"
    
    If the course is straight toward an object, such as a lighthouse, on which a
    bearing has been taken, then there will be no information available, from a
    running-fix, about its distance. Not until you hit the rocks that it marks.
    Those are not the conditions under which a running-fix is useful.
    
    =====================
    
    Where John Karl and I differ is with regard to his assertion that navigators
    should not even be taught such a necessary technique.
    
    John has not addressed the scenario I suggested in [11156], in which there
    was, effectively, no DR information available, and I request that he does,
    please. It's not an implausible state-of affairs. Indeed, without GPS, it's
    how a night-time approach from ocean to a new coastline, under overcast
    conditions, will normally present itself. Perhaps John is really confining
    himself to coasting, in which a DR estimate is more usually available.
    
    I will copy here again the relevant part of that posting, in which I
    suggested what seemed to me to be a plausible scenario, and asked John why a
    navigator should not learn how to cope with it.
    
    ============================
    
    As navigator, John has been travelling toward a coast under overcast skies
    for several days. He has little notion of his position.
    
    On a black night, with a bit of haze, a distant lighthouse is seen on the
    starboard bow, and identified by its pattern. Because of the haze, the light
    isn't at "dipping
    distance", but somewhat closer. It's impossible to know how much closer. A
    bearing is taken, and a line drawn on the chart. This is just a LOP. There's
    no reason to choose any one point in preference to any other, on that LOP,
    and mark it as an "estimated position". Any position, up to the dipping
    distance, is as possible as any other.
    
    John continues on the same course, and some time later, another bearing is
    taken of the same light, after there has been an appreciable change in its
    bearing.
    
    No other information is available.
    
    Will John discard the information provided by those bearings, because of
    some purist objection-on-principle to the notion of a running fix? Or will
    he estimate the course-and-direction travelled in the interim, allowing for
    any tidal stream as best he can, offset the first LOP correspondingly, and
    cross it with the second bearing?
    
    It doesn't bother me what name is given to that procedure; whether it's a
    "fix" or "not a fix". Under those circumstances, it provides the best
    position that can be obtained. Why should a student not learn it?
    
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    --
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