NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: On the integration of location and data
From: George Huxtable
Date: 2009 Nov 9, 13:59 -0000
From: George Huxtable
Date: 2009 Nov 9, 13:59 -0000
How splendid! Another top-grade, mind-blown rant from Frank Reed. We haven't had a really good one from him for some time, but this is one of his best. Replete with personal abuse, wild exaggeration, capital letters, full of sound and fury. His toes must have been firmly trodden on. It's given rise to some chuckles here, and no doubt in many households where Navlist is read. I do hope we aren't destined to relive the whole sequence of postings that arose in the thread "Learn the stars, by phone", back in May and June. Anyone interested can go back into that history, as I suggested. It will provide balance to some of Frank's wilder claims. Such devices, as with magnetic compasses, are only as accurate as the direction of the local Earth's field, with any perturbations, can be known. When the snags of using Earth magnetism as an azimuth reference were pointed out, Frank pointed to the precision claimed for the Skyscout, as 0.5 degrees. When that claim was questioned as implausibe, he replied, in [8275]- "That half-degree claim comes from the official specs. It's a believable claim, based on performance, but the exact level of accuracy (whether it's 0.5 or 0.75 or 1.0 degrees) is not critical in any way to the device's use." Well, I might have gone along with that, if indeed 1.0 degrees was the limit. But when it transpired that the tolerance had been relaxed to 3 degrees, without affecting Frank's enthusiasm, one wonders just how much imprecision Frank is prepared to accept. I hope he will tell us. Frank's response is this- "And just a reminder, George, you have still not even bothered to define "pointing precision". How strange to worry so much over something that you have not defined..." Nor, I might add, did Frank define it (in which he took refuge later). Nor did Celestron. All we can say is that however it was defined, it was later degraded to being 6 times worse than those initial claims, as relayed by Frank. However, none of this mattered, it was claimed, because such a device was capable of detecting the presence of local perturbation, and warning the user. I doubted whether the device would be able to detect sufficiently small perturbations, suggesting- "It could, perhaps, by detecting any significant difference in the total field strength, or the dip angle, from the value predicted for that location on the Earth. But could it establish, by such means, a deviation that changes the magnetic direction by 1degree? I don't believe it!" to which Frank responded- "I agree -- that wouldn't work." So, in its place, he speculated- "...measure derivatives of the field. The terrestrial field is quite uniform over considerable distances." The recent observations I reported, with a small bar magnet, demolish that notion. We're left with checking of the observed field against its predicted total value, and dip angle. Which, as Frank concedes, "wouldn't work". It doesn't. Not well enough. ====================== Frank recently pointed us to his posting in May, without alluding to the errors it contained. I reminded readers of those shortcomings, which seems to have triggered this explosion. I wrote- "In the absence of any magnetic perturbation, no doubt that instrument, or some sort of fancy-phone that works the same way, might do a reasonable job in pointing precision" Which drew this response- "Fine. That's all that's required. Let me repeat that a little louder for you: THAT IS ALL THAT'S REQUIRED. " But is it? How, in this World, where iron and steel are everywhere, can the user know that there is no local magnetic perturbation? Clearly, the warning-indication isn't trustworthy. The defects in such a device are not in the device itself, but in the magnetic field it's trying to sense to determine azimuth. 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 -~----------~----~----~----~------~----~------~--~---