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    Re: Sextant Positions versus Map Datums?
    From: Steven Tripp
    Date: 2002 Jan 17, 7:44 PM

    On 1/18/02 2:03 AM, "John Kabel"  wrote:
    
    > Trevor Kenchington wrote:
    ...
    >
    > 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.
    
    Japan has re-normed its charts starting 1/1/2002 to conform to GPS (WGS84, I
    guess) numbers.
    
    Nigel Calder has a good discussion of these issues in his new Cruising
    Guide.
    
    Various ellipsoids are optimized for their section of the surface of the
    earth.  For that reason no ellipsoid can be universal.  The surface of the
    earth is not a mathematical function.
    
    Each point on earth has an astronomical latitude and longitude, which is
    what you measure with a sextant and the almanac.  If you are near the center
    of the North American ellipsoid (or any other ellipsoid) there will be
    almost no discrepancy.  If you are at the edges there will be increasing
    discrepancy between your astronomical lat/lon and your charted position
    because the charts (and the ellipsoid)  will be in error.
    
    Calder points out that a certain amount of error is assumed (tolerated) by
    map makers because they know perfection is impossible.  I don't remember the
    exact numbers but it is in the range of 100 meters or so (it's actually a
    percentage). And this does not includes mistakes made by the survey team.
    
    Steve Tripp
    

       
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