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    Re: Sextant Positions versus Map Datums?
    From: Trevor Kenchington
    Date: 2002 Jan 16, 8:07 PM

    Peter Smith wrote:
    > Between datums, yes. The "horizontal difference" I was referring to
    > was between the ellipsoids that underlie the various datums. When you
    > project celestial LOP's onto either a sphere or one of these
    > ellipsoids, you get pretty much the same result. It's the landmarks
    > that appear to move when you compare charts built on different
    > datums
    I must be missing something here.
    As I understand it, the various datums are different ellipsoids, each of
    which is a mathematically-perfect surface which closely (but not
    perfectly) matches the shape of the real globe, the early ellipsoids
    differing in which part of the real globe they most closely matched.
    Surely then it is not the landmarks which move (they, after all, are
    rooted on bedrock) but the positions of latitude and longitude lines
    which are defined relative to the particular ellipsoid?
    If so, Jared's question seems a valid one. For which datum (i.e. which
    ellipsoid) are the tabulated numbers in the various sight-reduction
    tables calculated? Or are are they in fact (as I rather suspect)
    actually calculated on an assumption that the Earth is a perfect sphere?
    [Most of the numbers in the Almanac presumably relate the positions of
    celestial bodies to the centre of the Earth and so are not affected by
    the chosen ellipsoid. Or am I off-base on that?]
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                        Science Serving the Fisheries

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