NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Compass Error Corection
From: George Huxtable
Date: 2010 Jul 2, 14:16 +0100
From: George Huxtable
Date: 2010 Jul 2, 14:16 +0100
If you want to trace back into this series of postings, it helps to be aware that some have been spelled "Compass Error Corection", as is this one, and others, correctly, as "Correction". If you search under either threadname, you will find only some. Searching under "compass error" should find all. ==================== This is a response to a strange posting by Joe Schultz, on 1 July. It included the following statements- "...given up on this list (as do most practical navigators) ..." "...you're in the wrong place for combining innovation and practical navigation. This list has been taken over by (mostly) dreamers and historians, and you went right over their heads.", dismissing those "dreamers" at several points in his text. In which case, I wonder why Joe Schultz continues to grace our list with his presence, and his postings. How does he manage to overcome his distaste? ======================= As (I hope) one of that select category of "dreamers and historians", perhaps I have a bit of explaining to do, to Joe Schultz. The US Navy may well have a particular problem, in that every move a man makes is prescribed by regulation. Even when those regulations make little sense, as Byron implies is the case, it may well be a Herculean task to get committees to admit it and make changes. The rest of the seafaring world is less hidebound. I can't speak for the merchantmen, who know doubt have their own, less rigorous, rule books. But the recreational sailors ("yachties", in Joe's words) are free spirits, in this respect. They have the privilege of being able to apply common-sense to their navigation, and by and large, that is what they do. So, for example, this one-in-sixty approximation, for a sine or tangent of a small angle, about which much has been made by Byron and by Joe, is familar stuff to British small-craft users, even those who don't admit to knowledge of trig but still apply it to narrow triangles in chart plots. It's taught in all such evening classes, here in the UK, with the important proviso that it applies, approximately, to angles of only a few degrees, depending on the accuracy being called for. On this side of the Atlantic, the matter would be regarded as trivial: just as Apache Runner has implied in a recent email. And so, the linked file, provided by Joe at http://www.fer3.com/arc/imgx/1-in-60.gif, adds little information. Indeed, I've puzzled over that page for some time before taking in its message, because there's no explanation of what it is trying to do. He writes the equation sin A = U / D, which would be valid enough for a right-angled triangle, but then attempts to illustrate it with a triangle which is deliberately drawn to be very far from right angled, and to which that expression would certainly not apply. A fine recipe for sowing confusion in the mind of a reader. What's listed as "step 1" is not in fact a step in the calculation of error at all, but establishing a rule for minimum distance of a charted object to meet certain requirements, which happen to be met in one of his following examples, not in the other. And that criterion differs, considerably, from what Byron considered to be that minimum distance, in his posting of 13 April, when he wrote- "All that is required is to estimate your position alongside the pier, determine how accurate your position is known + or - 10, 15, 20 yds etc. Once this value is known double it and multiply by 60 ‘ result is the minimum distance you want an object to be in which to obtain a bearing to". Byron didn't state, there, what bearing accuracy this test was expected to provide, but further down that page it turns out to be a quarter of a degree. Indeed, if a quarter-degree was intended, shouldn't Byron's "double it" really be "quadruple it"? However, this depends on whether the position inaccuracy is defined as how far from the presumed position the observer may be, or alternatively twice as far; the overall range of possible positions, between one direction and its converse. Byron doesn't say, and Joe hasn't clarified that ambiguity. ========================= Joe Schultz then presumed to offer Byron advice, including - "Be careful with the numbers you use in your examples, so you don't inadvertently confuse the reader" Which is a bit rich, considering his own linked file. And continued- "Write for your audience. The combined underway fix and compass error can't be done by merchies/yachties, for example, unless an athwartship range is shot while steering another range". He would do well to follow his own advice. Who make up this audience? One is me, a native speaker of English, of a sort, though a rather different sort to that of Joe and Byron. Others on this list are not native-speakers of English at all, but manage well enough to put many of us to shame. But I, for one, have to guess at Joe's meaning when he writes- "...an athwartship range is shot while steering another range." Does a "range" mean the same as an azimuth, I wonder, or a direction, or a course, or what I would know a "transit"? Where I was brought up, a range is some sort of measure of distance, not direction. And what on Earth are "ring knockers and trade schoolers"? Two contributors to this list, who give me most difficulty in interpreting what they write, are Byron Franklin and Joe Schultz. Both, it so happens, are US Navy trained. Is there something special about US Navy language, larded as it is with acronyms, jargon, and homely folkiness, which makes it hard to follow? Do others have the same difficulties that I do? 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.