NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: typical standard deviation?
From: George Huxtable
Date: 2009 Sep 19, 23:28 +0100
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. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---