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    Re: Accuracy of position
    From: Geoffrey Kolbe
    Date: 1999 Oct 21, 1:44 AM

    Thankyou George Huxtable for your comments.
    I think you have put your finger on the nub of my question. The refraction
    from the horizon is considerable and accounts for about 20% or so of the
    height-of-eye or dip correction. So while the height-of-eye may be greater
    on a supertanker than for a yacht bobbing about in the waves, leading to a
    flatter horizon, the corollary of this is that the refraction from the
    horizon is greater for the supertanker, leading to a greater potential for
    error due to abnormal refraction from the horizon.
    For small vessels then, the principle error may be the uncertainty of the
    horizon due to the fact that it is not flat and that the height-of-eye may
    not be constant (due to bobbing about in the waves!) For large vessels,
    height-of-eye is not a problem, but as the horizon is now further away,
    problems of abnormal refraction could start to dominate, particularly where
    there is a large difference between the water temperature and the air
    temperature, and/or where the air is still. Too, the horizon may be
    indistinct for any number of reasons.
    I should come clean and say that I have very little personal experience of
    using a sextant for navigation at sea. My original question, regarding the
    uncertainty you would place on an altitude reading using a marine sextant
    at sea, was really aimed at the old sea salts who have used a sextant to
    keep themselves found at sea and came to know from experience just how much
    precision they could rely on from their sightings.
    So, the sextant is a Plath and it comes with a certificate saying the arc
    is good to 6" of arc. You take what seems to be a perfect sighting of a
    body on a clear day and make the necessary correction for dip and
    refraction and come up with an observed altitude of, say 55 degrees 26.3
    minutes. Do you say, "Right, that is what it is." Or, as an old sea salt do
    you say, "That's all very well, but in my experience, the actual altitude
    may be up to x' either side of that, so I will just keep that in mind when
    plotting my position."
    The question to the old sea salts is, how big is x?
    Thanks again,
    Geoffrey Kolbe

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