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    Re: Sextant Positions versus Map Datums?
    From: John Kabel
    Date: 2002 Jan 17, 10:03 AM

    Trevor Kenchington wrote:
    > 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?]
    John Kabel replies:
    I have looked through both my Nautical Almanac and the 1992 edition
    of the Explanatory Supplement to the Astronomical Almanac.  In
    neither is there any mention of adjusting for datum after a sight
    The NA is referenced to the plane of the equator and the longitude
    line through Greenwich, England.  All "predictions" for star and
    planet positions are referenced to these entities.  All astronomical
    effects of inconsistency of earth rotation, polar wobble and the
    effects of solar system bodies on each other's orbits are taken into
    account and "buried" in the tables.  The system appears to be Earth-
    centred by definition.
    I would suggest that in the past, locations of foreign datums were
    determined by celestial means, relative to Greenwich and the equator.
    All of the systems of modern surveying originated in Britain and the
    United States, which have agreed on the basics as far back as
    practical legal surveying has existed.
    So the latitude and longitude on charts in a particular area will be
    already be historically referenced to the standard on which the
    Almanacs are based.  If sailing near Japan, for example, one would
    already be doing the DR on charts in the Tokyo datum, and working up
    plotting sheets based on those charts.  One would not be using a
    chart based on NAD-27, unless you were really missing the point of
    safe navigation.
    Any suggestion that you were doing sights based on a chart referenced
    directly to Greenwich and the equator would mean that you were trying
    to re-survey Japan, in my example.  That is meaningless, and I doubt
    the locals would appreciate it.
    Page 21 of my 1995 Bowditch also discusses Datum Shifts, especially
    the problem of moving between charts with different datums.  "If any
    positionis replotted on a chart of another datum using only latitude
    and longitude for locating that position, the newly plotted position
    will not match with respect to other charted features.  This datum
    shift may be avoided by replotting using bearings and ranges to
    common points.  If datum shift conversion notes for the applicable
    datums are given on the charts, positions defined by latitude and
    longitude may be replotted after applying the noted correction."
    Bowditch also gives four suggestions for minimizing errors caused by
    different datums on the same page.
    Again, I suggest that an attempt to place all one's sights in a
    global reference is meaningless, since you practically have to adjust
    onto new charts as you move around the world.  Sights are relative to
    DRs in the local datum, not absolute, at least not at our level.
    John Kabel
    London, Ontario
    43d 01.177'N , 81d 12.089'W, give or take 10 m.

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