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    Re: Sextant Positions versus Map Datums?
    From: UNK
    Date: 2002 Jan 15, 12:05 PM

    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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