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    Re: Sextant Positions versus Map Datums
    From: John Kabel
    Date: 2002 Jan 18, 6:58 AM

    [This was part of an answer to Trevor Kenchington yesterday.  I'm
    sorry if the message is a repeat.  I never got a bounce from the
    list, so I am not sure it got through.  My ISP is having BIG problems
    right now.]
    
    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
    reduction.
    
    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,
    probably with instruments with proper levelling attachments, so they
    would be vertical to the geoid.
    
    So, all the 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.
    
    Page 21 of my 1995 Bowditch also discusses Datum Shifts, especially
    the problem of moving between charts with different datums.  "If any
    position is 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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