Hi all,
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I must come down squarely on Jeremy’s side on this
one, but do remind that I have repeatedly preached
the limitations imposed upon the ocean navigator by the condition of the
sea horizon – other than inadvertent errors in
reading off the sextant or chronometer, which can to a degree be
controlled by care, it is probably the most consistent cause of position
error or non-conformity among a group of sights. Haze, glare, cloud
shadow, low lying clouds, sand storms, abnormal temperature gradients,
etc., all militate against the navigator, especially if unequally
distributed as respect a round of observations.
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I missed this Henry, but I am
glad that I am not alone in this observation.
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It has always been considered appropriate to observe
stars, or even the sun at high altitudes permissive of back observation,
on reciprocal bearings, i.e., N – S and E - W, or as otherwise
convenient, so as to cancel out both horizon and sextant errors and, in
the case of stars, to produce a box like fix, the size thereof being
indicative of the uncorrected errors involved. It was
my custom, for what it’s worth, to observe a round of at least 6-stars,
if available and conditions permissive, at 60-degree intervals to
produce a fix – obviously this is not always possible, however, I do
think it important to at least try to avoid taking all observations in
one quadrant, or even semicircle.
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I always take stars from
around the compass if at all possible. I shoot the pre-calculated
stars from HO 249 plus any planets and/or the moon. If I have
time, I may even shoot a star not on the list. I will also shoot
"2 rounds" of the brigher stars if i can swing it; once at each end of
twilight and compare them for errors. With a good sky and horizon
i can be reducing 7-14 LOP's for a fix. If there are clouds or a poor
horizon, I shoot what I can and expect less absolute position
resolution.
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s/s African ___,
New York to Capetown, Voyage 3: True calculated Great
Circle Distance = 6.764 nm; actual miles steamed by celestial navigation
= 6,798 nm; difference = +34 nm.
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s/s African ___,
Capetown to New York. Voyage 4: True calculated Great Circle Distance =
6,764 nm; actual miles steamed by celestial navigation = 6,780 nm;
difference = +16 nm.
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s/s African ___,
Capetown to Port of Spain, Voyage 3: True calculated distance by
combination Great Circle + Rumb Line = 5,358 nm;
actual miles steamed by celestial navigation = 5,380 nm; difference =
+22 nm.
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These are actual
records of what I believe to have been a carefully navigated vessel
solely by celestial navigation. Certainly they do not assess individual
sight quality, but is it possible that such records might be used in
retrospect to assess the overall quality of the navigation method
employed? If so, they are potentially available in historical voyage
abstracts, probably numbering in the many thousands.
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I find this
interesting, and quite good for efficiency, but it still does not help
really with the kind of statistics we are talking about. When we
are looking for resolution in fractions of an arc-minute, knowing GC
sailing efficiencies isn't going to go too far. I know for a fact
that with anything close to decent conditions I can guarantee my star
fix to be within 2 miles of actual position, and usually well under 1
mile. When we want to figure out standard deviations of groups of
sights, we need lots of empirical data compared to a datum like GPS so
that we're all starting on the same page so to speak.
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There is no
question but that some navigators were professionally enough interested
in their skills to test their ability by observations for position in
known locations. Were records made of such testing results, other than
perhaps in personal diaries or workbooks? – certainly not, as there was
no Internet, and no NavList, available as today for the instant and
public exchange of information. I have posted on this List examples of
the accuracy to be expected from various celestial navigation
observations, as compared with known positions, always stating horizon
conditions, sextant employed, and method of reduction; these postings
have been casually accepted, generally without comment.
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I need to do a
search for these posts, as I am keenly interested to compare my results
with these.
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Quite
frankly, I do not believe there to be a great deal of difference in
observing from your front porch, given that you have a view of the
horizon, or the stable platform provided by a large ship in all but bad
sea states; in fact, it may be easier from the ship, as a close inshore
horizon seems more frequently hazy. With the instruments available to
members of this List, it does not appear difficult that a project,
leading to the establishment of a data base of observational error
against know position, might well be undertaken.
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however, have 2 bound workbooks, totaling over 500 pages of closely
packed, worked observations and related calculations done in the late
1940s and early 1950s, before going on active naval duty, for which I
might be pleased to find a secure home. Copying is out of the question
for me, as all the work was done with a 2-1/2 hard pencil and, although
perfectly legible, too light for the conventional copy equipment to
reproduce clearly.
----------------------
I'd love to see
these, and am curious as to what methods of reduction you used. If
my history serves me well, that would have been HO 214 days, but I am
not sure how well distributed these tables were at that time.
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Regards,
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Henry
---
On Wed, 8/12/09, Anabasis75@aol.com
<Anabasis75@aol.com> wrote:
From:
Anabasis75@aol.com <Anabasis75@aol.com> Subject: [NavList
9479] typical standard deviation? To:
NavList@fer3.com Date: Wednesday, August 12, 2009, 2:35
AM
George wrote:
"The deduced scatter, of one standard deviation about the fitted
trendline, I now make to be 0.53 arc-min in the case of the "Moon
near LAM" set, and 0.38 arc-min in the case of the "Moon away from
LAM" data set. That's no better, and no worse, than one would
expect from observations at sea from a large vessel."
I wonder on what basis this statement is made. I am not
claiming to be any better or worse, than any other practiced
navigator, but how can we know somewhere around 0.5 arc-min is what
"...one would expect from observations at sea from a large
vessel." Is there some collection of data that would back this
up? I am just wondering how we can expect this kind of deviation
from large ships as opposed to small vessels unless there has been
some sort of study on ships of various sizes and under different
observing conditions or a review of a variety of navigational
logs.
Can any other navigator on this list give data that would support
this kind of statement, even if the data isn't recent?
The reason I bring this up is that as I look through my recent
navigational log, I notice that the most critical aspect of shooting a
star is seemingly never mentioned (at least far less than sea state,
large ships, and anomalies in dip). This factor is the
quality of the visible horizon! The horizon varies with
time, azimuth, and circumstance every time I shoot. I have shot
with a crisp horizon in one direction, and a fuzzy horizon in another
quadrant at essentially the same time. The horizon's
quality at a given azimuth, affect my sights far more than any other
shooting condition. The quality of the horizon greatly changes
the accuracy of my star fixes which is the only measure I truly care
about.
Given the nearly endless variations of observing circumstances
for any sight using the visible horizon rather than a bubble or other
artificial horizon, I find it hard to justify stating in all but broad
terms what magnitude of scatter we can expect from a given type of
vessel.
Jeremy
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