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    Position Lines: was; Problems with AstronavPC
    From: George Huxtable
    Date: 2004 Feb 17, 21:35 +0000

    Jim Thompson said-
    >I received an email from a
    >non-navigator amateur historian (recall that I am a very amateur navigator).
    >He wanted to find the coordinates of an historical event that occurred on
    >land in the 19th century.  He had the altitude of a body and the time.  I
    >approximated the position by systematically entering alternative data into
    >an online CN calculator, but that was pretty crude.  In the end it turns out
    >that his altitude/time information were too imprecise to be useful anyway.
    From George-
    That attempt was doomed to fail if it was trying to locate a point. All
    such a single observation could do was to define a position line, part of
    an enormous circle on which the observer must have been located.
    he went on-
    >I understand that the LOP we plot actually is a tiny segment of a huge COP
    >(Celestial Line of Position = "CLOP", if one wants a new acronym), but if
    >the computer was able to return (or we were able to calculate):
    >1. The lat/long coordinates (EP) for the intercept of the azimuth with the
    >LOP (COP or CLOP), and
    >2. The azimuth of the GP,
    >then all we would need to do on an ordinary plotting sheet is:
    >1. Put a dot at the intercept,
    >2. Place square box around it (EP symbol),
    >3. Draw a short segment of the azimuth from the intercept (EP) toward the
    >4. Draw the LOP as a straight line perpendicular to the azimuth.
    >I realize that this generally is not good practice, because we should be
    >keeping a tight DR plot to complement the CN sights, but is this not a
    >realistic alternative to using an AP?
    If the azimuth of the body seen from the observer was accurately measured
    (and that can be done much more precisely from on-land than from at-sea, if
    one has some sort of theodolite and a precisely surveyed North-South line)
    then in princple, that can define where on the position circle the observer
    must lie. However, I'm not sure how to find that point using simple
    geometry. And it's VERY demanding on measuring an accurate azimuth, if the
    GP of the body is thousands of miles distant, and the position is required
    to a mile or so. Impractically demanding, I suggest.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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