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    Re: typical standard deviation?
    From: Gary LaPook
    Date: 2009 Aug 12, 14:40 +0200
    Jeremy asked:
     
    "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?"

    I wouldn't expect to find such data. During the history of celestial navigation IT was the most accurate navigation system available so there was no other data that the celnav positions could have been compared with to develop such an accuracy data set. Really, only since the development of GPS (maybe LORAN C) has such another set of positions become available so only now can such data be developed. But how many navigators continued to use celnav on a regular basis and then logged the accuracy of the celnav position derived after comparison with the contemporary GPS positions and then made their logs available.

    Jeremy seems interested in this and does continue to use celnav on his large vessel so maybe he will be able to provide such a data set as time goes by. I have a project to develop one such set of data. On October 22nd we sail from Lisbon for Barbados on the Royal Clipper, a five masted sailing ship. The passage will take 16 days with stops in Morocco and the Canaries. I plan to take my sextant with me and a GPS and will be taking morning and evening stars as well as morning and afternoon sun sights and will compare the fixes and LOPs with GPS positions. If internet is available on the ship I will send the data to the list as I go along otherwise when I reach shore. The ship is 439 feet long so I guess it counts as a large ship for celnav purposes.

    But, even then, we will only have a small data set by one observer (so probably not the best observer in the world.) If others on the list have maintained such logs of accuracy of celnav compared with GPS then maybe they could also share their results with the rest of us.



    see:  http://www.starclippers.com/ships_rc.html

    http://www.starclippers.com/gallery/tours/english/video_eng.htm


    gl



    gl
     





    Anabasis75@aol.com wrote:
    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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