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Typical standard deviation?
From: Jeremy C
Date: 2009 Aug 12, 02:35 EDT
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From: Jeremy C
Date: 2009 Aug 12, 02:35 EDT
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."
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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