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Subject: Re: Sextant Accuracy and anomalous dip.
From: George Huxtable (george@XXX.XXX)
Date: Mon Mar 17 2003 - 14:53:20 EST
Fred Hebard wrote-
>The location of my house seems to be settling down to perhaps 0.3 to
>0.4 nautical miles north of its actual location. All of this with an
>artificial horizon. One new technique I tried was to preset the
>sextant to the future altitude of a body and then watch the body pass
>through that altitude while listening to the time being counted out.
>
>My general objective is to get these observations as accurate as
>possible, say to within 0.1 or 0.2 miles. I'm not sure why I have this
>objective, but it persists.
>
>Bill Murdoch mentioned he had never bothered to apply instrument
>corrections to readings taken with his Freiberger, implying that it's
>not necessary to achieve accuracies better than about 1 nautical mile.
>That also seems to be the general opinion here, especially with regard
>to observations from small yachts, where it often is not possible to
>get more accurate. But how about larger vessels? Does anybody want to
>defend trying to get accuracies closer than 1 mile?
Comments from George.
Fred has every right to be satisfied with these results. Presumably he has
rather sharp eyesight. It would be interesting to know how much scatter
there is between Fred's individual measurements of the position of his
house. Is his result of "perhaps 0.3 to 0.4 miles Morth" an average of many
observations, perhaps? I'm interested in how closely they cluster about
that value.
I am presuming that the artificial horizon used is some form of reflecting
pool. Altitude measurements made that way on land have several advantages
over horizon altitudes measured from a boat. Because the doubled altitude
is measured on the sextant, the effect of sextant calibration (and other)
errors is halved. With a stable platform underfoot, the observer isn't
being buffeted and his images are steady, so he can use a high-gain
telescope. If a glass cloche is being used as a windbreak, care has to be
taken over the optical quality of the glass panes used.
The main errors in measuring an altitude at sea arise from deficiencies in
the sea-horizon that has to be used as a reference: deficiencies that are
absent in artificial-horizon observations on land.
From a small boat at sea, the horizon is not a steady straight line but a
succession of overlapping wavecrests, observed from a platform that's also
heaving up and down, changing the perspective view of the waves and
altering the dip from moment to moment. A big vessel, with its slow and
predictable roll, and its high viewpoint, is MUCH less affected in that
way.
Even in millpond conditions, however, when the small boat loses much of
that disadvantage, there remains a source of error affecting the
horizontality of the horizon, for small vessels and large ones alike. This
is an old "hobby-horse" of mine, and I hope nobody minds if I trot it out
for a ride once again.
This error is "anomalous dip", when the air-layers within a few feet of the
horizon differ in temperature in an unexpected way, and refract
accordingly. It causes dip to vary slightly, in an unpredictable way, and
it's hard to detect it happening, let alone estimate how much to allow for.
This effect has nothing to do with the small corrections for refraction,
depending on temperature and pressure, which most almanacs provide.
Changes of dip of a minute or so from the predicted value can be quite
frequent, changes of 2 or 3 minutes can occasionally happen, changes of 5
minutes are possible though very unusual. These can perturb a sextant
altitude by a corresponding amount, and the observer will be completely
unaware that it has happened.
So if you are a sextant-navigator wishing to avoid a dangerous hazard, how
much room do you allow to consider your vessel safe? Usually the dip-error
will be within a minute, but it's hardly satisfactory to say that "usually"
you will miss that dangerous rock. You wouldn't wish to navigate in such a
way as to hit it "only occasionally". A safety-standard of "almost never"
is I think the very minimum anyone would accept, and any prudent navigator
would insist on a better margin even than that. So a navigator without
knowledge of the dip-of-the-moment might be wise to allow a margin of 5
miles or so, to cater for possible anomalous-dip.
Dipmeters have been invented and used in oceanographic survey work. A crude
but effective dipmeter can be knocked together without great expense. It
requires a view of the horizon in two opposite directions to be visible
simultaneously from somewhere on deck.
There may be a case for a quick check of the dip as a matter of routine
whenever sextant observations are made. The results would probably be very
boring. Most of the time the dip-error would be well within one arc-minute.
But on those rare occasions when it was considerably greater, the navigator
would be forewarned, and could act to correct it. That correction would
allow him a greater reliance on the precision of his sextant observations.
Anomalous dip seems to be generally disregarded as a hazard to sextant
navigation. However, I think it's worth taking seriously. There's more
stuff about anomalous dip in old archives of this mailing list.
George Huxtable
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contact George Huxtable by email at george@XXX.XXX by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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