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Subject: Re: Sextant Positions versus Map Datums?
Smith_Peter@XXX.XXX
Date: Tue Jan 15 2002 - 14:04:41 EST
On Tue 15 Jan 2002 1:05 PM in Navigation-L,
Jared Sherman [mailto:jared.sherman@XXX.XXX] 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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