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Subject: Re: Accuracy of position
From: George Huxtable (george@XXX.XXX)
Date: Thu Oct 21 1999 - 16:00:04 EDT
Just to make a quick comment on Geoffrey Kolbe's mailing earlier today-
I agree with all he says, but wish to point out that as the dip, and the
refractive component of the dip, depend on the square root of the height of
eye, the difference in the horizon refraction between a small vessel and a
large one is not as great as one might first imagine. The figure I quoted
for the average refraction to the horizon, for a small boat with a 10-foot
height of eye, was 0.6 minutes. From a 40-foot height, it would only be
twice that, 1.2 minutes. A significant increase, but not an overwhelmimg
one.
As for his challenge to estimate the value of x, the maximum error in a
good sextant altitude observation, it's not easy to give an answer. In a
small boat in rough weather, the error just increases up to the point where
sextant sights become impossible, and the navigator shuts the hatch and
cowers below. In calmer conditions, I would put it at 2 to 3 minutes if a
really sharp horizon is visible, even with my plastic sextant. But I would
keep in mind that occasionally, due to very abnormal refraction
(unrecognised) at the horizon, the error could be greater.
The big test would come if I was heading for a narrow rocky passage, 6
miles wide, in the dark, from a position defined only by astro observation.
Would I trust my life to the assumption that any error would be less than
three miles? The easy answer, to pass the buck, is that I would get out my
GPS.
However, I must admit that I don't go ocean sailing.
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.
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