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Jared Sherman said-

>Paul-
> practiced, provides an average error in position of 2 nm.>
>
> You don't know HOW GLAD I am to see that from that source. Here I was,
>getting 2 to 2.5nm errors on dry land and saying to myself I ought to be
>able to get closer than that.
>
> Which opens the other topic...Is that "good enough for government work"
>or simply as good as it gets?

================

In the past, this list has gone into the question of attainable precision
of sextant observations more than once, and has shown somewhat divergent
views.

Since then, I have become aware of a paper by Captain Henry H Schufeldt,
USNR, entitled "Precision Celestial Navigation Experiments", in the
"Journal of the Institute of Navigation", vol 15 (1962), pages 301 to 324.
This is the London journal, not the US one, and since its renaming might
well be kept in libraries under "Journal of the Royal Institute of
Navigation".

This article is so relevant to the interests of many on this list that I
think it should be better known.

Schufeld has gone to a lot of trouble to minimise errors, for example by
trying higher-magnification telescopes, and has found that mag of x20 gave,
in some circumstances, significantly improved accuracy (in his warship
environment, which is very different to our small craft).

Another important factor he could point to was variation in dip from the
predicted value, which he was able to correct from the readings of a
Gavrishell dipmeter. Near Bermuda, tests showed that without the dipmeter,
the mean distance from the known position was 0.85 miles, but taking
account of the measured dip reduced that to 0.115 miles. He reports, on
another occasion, a daily variation of dip, being one minute greater in the
afternoons and evenings than in the mornings, which lasted over several
days.

Schufedt's final conclusion is this- "... it is held that a ship's position
may be fixed at sea, under good observational conditions, to about 0.25
miles, by a round of multiple observations of stars, made just before
sunrise or after sunset, with a high quality sextant, fitted with a 20x
telescope. This conclusion is based on the results achieved during the
course of this study. Remote read-out of sextant altitude and time should
improve this somewhat. With 6x and 3x star telescopes, it has heretofore
been possible under similar conditions to obtain an accuracy of about 0.4
miles"

But I recommend you read this paper for youself. If you find it's
impossible to obtain a copy, and ask me nicely, I might well post off a
photocopy to you.

I am very interested in Schufeldt's mention of a Gavrishell dipmeter. I
have never heard of this instrument before, though I have heard of other
dipmeters by Carl Zeiss and by Blish. Has any listmember heard of the
Gavrishell instrument?

Jared Sherman is observing from onshore, so he has the advantage of an even
more stable platform than Schufeldt's. He is, I presume, measuring up from
a sea horizon. In a coastal environment, I think it quite likely that
variations in the dip of the horizon might well be greater than out at sea,
because of airmasses at different temperatures mixing near the shore.
That's just my guess.

However one looks at the Schufeldt paper, there's a big difference between
the accuracy of 0.25 to 0.4 min  that he quotes and the 2 min that's
mentioned in Paul Hirose's document. What was the 2 min referring to, I
wonder? Did this relate to astro measurements made in the air, using a
bubble sextant?

For those of us that sail our small craft out at sea, my opinion is that if
we can achieve a precision of 2 min, we are doing pretty well. What do
others think?

George Huxtable.



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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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