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    Typical standard deviation?
    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
    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.

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