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    Re: Sextant Positions versus Map Datums?
    From: John Kabel
    Date: 2002 Jan 15, 4:26 PM

    For the group's interest, there is a diagram comparing positions
    obtained using GPS at various datum settings.  Now, I know this list
    is not about GPS, but the diagram is instructional, using the fairly
    accurate positioning capability of the GPS to demonstrate  the
    effects of datum conversions.  It is part of a thorough discussion of
    datum effects by Peter Dana:
    It is worth comparing some of the datum-datum differences to the
    error we would typically see with celestial navigation (usually
    considered to be more than a nautical mile, perhaps as much
    as 5-10 nm depending on conditions).
    P/Cdr John Kabel, N
    GPS Instructor
    London Power and Sail Squadron
    London, Ontario
    > On Tue 15 Jan 2002 1:05 PM in Navigation-L,
    > Jared Sherman [mailto:jared.sherman@VERIZON.NET] asked:
    > > Okay, a sextant position is based on the almanac which is based
    > > on...ergh...How exactly does that match up to a map datum? Should
    > > a sextant position line up with a WGS84 chart position? An NAD27?
    > > A what-ever-is-most recent datum? Or can it be expected to be off
    > > by...What distance actually?
    > ...
    > Your celestial fix gives you a latitude and longitude, which you can
    > plot on a plotting sheet, or on a chart with equal validity (*).
    > However, on a chart, you've got all these shoals and harbors and
    > buoys and other things. Their latitude and longitude on the chart
    > depends on the horizontal datum (NAD-27, WGS-84, etc.) that was
    > used by the chart maker. Chart datums are sets of reference points
    > which are assigned latitude and longitude, and all other landmarks
    > are charted with respect to these.
    > So, the issue when you plot a celestial fix on a chart is not
    > where you are, but how to steer from the fix to arrive at or avoid
    > some other landmark, whose position depends on the datum.
    > (*) OK, the various datums use slightly different ellipsoids, but I
    > believe that the horizontal difference is not significant for
    > this discussion. The difference is more evident in the vertical
    > distance between the sphere and the ellipsoid, and the orientation
    > of the vertical. True?
    >  -- Peter

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