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    Re: typical standard deviation?
    From: Jeremy C
    Date: 2009 Sep 17, 19:52 EDT
     
     
    In a message dated 8/16/2009 1:30:37 A.M. Central Asia Standard Time, h.halboth@yahoo.com writes:

    Hi all,

    <!--[if !supportEmptyParas]--> <!--[endif]-->

    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.

     

    --------------------------------

    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.

    ------------------------------

    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.

    -------------------------------

     

    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.

    <!--[if !supportEmptyParas]--> <!--[endif]-->

    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.

    <!--[if !supportEmptyParas]--> <!--[endif]-->

    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.

    -------------------------------

     

    <!--[if !supportEmptyParas]--> <!--[endif]-->

    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.

     

    ---------------------------------

     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.

    <!--[if !supportEmptyParas]--> <!--[endif]--> I do still, 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.

     

     

    <!--[if !supportEmptyParas]--> <!--[endif]-->

    Regards,

    <!--[if !supportEmptyParas]--> <!--[endif]-->

    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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