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    Re: typical standard deviation?
    From: George Huxtable
    Date: 2009 Sep 19, 23:28 +0100

    Henry wrote, responding to Jeremy, in [9804]-
    
    "Thank you for your comments on my previous posting 9538. You were the sole 
    List member responding thereto - somewhat of a disappointment to me, but 
    that's life."
    
    I at least, read that message of Henry's carefully, and appreciated the 
    sense and experience within it, particularly in his comments about horizons. 
    It didn't get a response from me, because I could see little to argue with 
    (but see below).  I hope he will continue to offer such postings; they are 
    valuable, and valued by his readers. Well, by me, for one.
    
    As for his intention "I shall see if I can make some legible copies of 
    selections from my workbooks. As you are the only one interested, there is 
    no point in boring the List so, if successful, I will contact you further to 
    arrange a possible Off List exchange.", I hope he will think again. There's 
    no need for any reader to be bored. Those that are uninterested can always 
    press the delete button. Henry, please share any such information with the 
    rest of us!
    
    By the way, image-processing software, such as is available in Corel 
    photo-paint (and presumably in Adobe photoshop), can do a lot to bring up 
    the contrast in a faint document, if the initial scanning is done with some 
    care, and the background is clean.
    
    ====================
    
    However, I do have a couple of comments about Henry's earlier message 9538, 
    though somewhat belated.
    
    He wrote-
    
    "Distance actually steamed, as compared with true rumb line or great circle 
    distance between ports of call, it seems to me, also can form a basis for 
    assessing navigational accuracy, or perhaps more correctly efficacy. Some 
    ships were kept very close to the intended track by frequent celestial 
    fixes, thus minimizing steaming time and distance, while others were 
    suffered to wander aimlessly to either side by sloppy navigational 
    practices, excused by the allegation of an “unpredicted or unknown current”. 
    Let’s take a couple of examples of what I mean .....
    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."
    
    ==============
    
    I wonder whether that change in distance provides such a good measure of the 
    navigational precision at all. The mid-point of that great circle voyage is 
    somewhere near the  point N6º 30', W24º. But if, instead of passing through 
    that point, the vessel were to make a diversion, to sail instead via a 
    waypoint at N1º 30', W30º, that would increase the journey length by only 35 
    miles or so, if each leg to that mid-point was sailed as a great-circle on 
    its own. Similarly, if the diversion was the other way, to pass through N12º 
    30', W18º, the length of the passage would be increased by around than 44 
    miles. Those alternative waypoints cover a span of ocean nearly 1000 miles 
    wide. Which implies that one can happily wander off-course, in mid-passage, 
    without increasing the journey-time significantly. The important thing is to 
    keep the vessel's head pointing in roughly the right direction, but there's 
    a lot of tolerance.
    
    ======================
    
    The other passage I would like Henry to expand on a bit is this one-
    
    "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."
    
    I accept Henry's experience from large ships, which I lack. Presumably, here 
    he is thinking of a modern vessel of many thousands of tons, and taking 
    sextant observations under appropriately favourable conditions. But I invite 
    him to consider, as well, the problem facing mariners in the days when they 
    were entiirely dependent on their observations to discover where they were, 
    no matter what the weather happened to be throwing at them, in wooden 
    sailing craft of a few hundred tons. Or the difficulties that face 
    navigators of today's really-small craft, even in favourable weather, in 
    their attempts to use celestial navigation.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 
    
    
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