A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: QMOW Days work in Navigation
From: George Huxtable
Date: 2010 Jan 10, 15:24 -0000
From: George Huxtable
Date: 2010 Jan 10, 15:24 -0000
I suspect Peter Hakel and I are at cross-purposes here. There seems to be misunderstanding, rather than disagreement, between us. He wrote, about taking a series of Sun altitudes to determine Local Apparent Noon- "However, according to the procedure you were supposed to do these noon-curve measurements anyway, so you're not doing any extra observational work beyond what is already mandated." Presumably, Peter here is thinking of the "procedure", "mandated" by the instructions quoted by firstname.lastname@example.org, in "Noon Observe LAN. Recommend observations be started at 10 minutes before computed time of LAN, and for a couple of minutes after. Reduce sighting and determine ship's latitude." But observing over such a short period as 12 minutes would provide no useful information about time of meridiam passage. To obtain that requires a number of observations, to maximum precision, spread over a period of an hour or so, symmetrically about the peak altitude, as has been discussed at some length on this list. That operation has never been part of a mariner's repertoire, as far as I know. I've not seen such a procedure recommended in a textbook, nor come across it referred to in use, in any navigator's logbook. If Frank's perusal of old ship's logs has shown up any such record, no doubt he will tell us about it. Near noon, if a mariner knew his GMT, and could estimate his longitude reasonably well (by dead-reckoning from a morning Sun position line, say), he could predict the moment of LAN and measure Sun altitude at that moment. He might take a few observations, closely-spaced, and average them to increase accuracy, but that's all. From that observation at that moment , he could derive latitude, without having to allow for North-South speed component. That appears to be the intention of the precedure quoted by email@example.com. On the other hand, if, as in early navigation, no chronometer was carried, or if it was distrusted, or if the longitude couldn't be estimated, the operation would have to be somewhat different. A series of altitudes would be taken near noon, increasing then decreasing, and the maximum altitude noted. This was done solely in order to acquire the VALUE of that maximum altitude to determine latitude; not its TIME. For precision, that latitude might call for a correction for North-South speed component, though in mid-ocean that would often be neglected. This question, about finding time of meridian passage from a prolonged series of altitude observations around noon, has become something of an obsession among some, here on Navlist, but this quest has never been of importance to the wider community of practical navigators. There were better methods, and they were aware of them. As for the "running fix", that;s a technique about which an aura of mysticism and distrust has recently been woven here by John Karl. But it can be a precise business, and is used a lot in celestial navigation, to link observations made at different times. The dead reckoning between observations is affected by unknown local currents, but in ocean travel, those are seldom greater than half a knot, and where they are, mariners are usually aware of them and can estimate them. So errors from that source, over a 6-hour interval , should not be greater than 3 miles or so. With care, a ship's speed through the waterso , even under square rig, could be estimated to within 10%, or thereabouts. Even in my own small sailing craft, which on a good day could do all of 5 knots, in the swirling waters of the English Channel with tidal streams which were often well over half that speed, my dead-reckonings would seldom be out by more than 4 or 5 miles, over a 6-hour "tide". However, the procedures being referred to were for a modern warship proceeding under power in well-charted waters, of which ocean currents would be predictably known, and dead reckoning of her progress should be highly predictable. With Sun altitudes morning , noon, and afternoon, linked together by a running fix, precise postions would result. George. contact George Huxtable, at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "P H"
To: Sent: Saturday, January 09, 2010 7:20 PM Subject: [NavList] Re: QMOW Days work in Navigation | Perhaps I should clarify why I found this detail so interesting. There are two reasons, in fact. | | | First, it is the extensive use of the running fix which has been both criticized and defended quite recently on this list. | | Second, less recently some of us (Jeremy, Antoine, Andres,...) went through some real-life meridian transit data that resulted in longitudes that were better than mediocre, in my opinion. The construction of the noon or any other meridian transit curve indeed "require extended and precise observational data" as George said. However, according to the procedure you were supposed to do these noon-curve measurements anyway, so you're not doing any extra observational work beyond what is already mandated. Before computers the "noon curve longitude" determination would result in non-trivial amount of extra computational work, which I suspect as the main reason why this was not done traditionally. This led to my hypothesis that accuracy may not have been the issue; the extra labor required was, hence the running fix instead. | | Now in 2010 we do have powerful computers and automatization of these calculational chores by software, essentially eliminating the extra labor problem caused by vessel's motion, for instance. This brings me back to the question of accuracy and also to my reason #1. If this "day's work procedures" were to be followed, wouldn't it be better to recommend "longitude by LAN" (which is a true fix) instead of doing the running fix using the DR advanced mid-morning Sun LOP and LAN latitude? | | | Peter Hakel | | | | | ________________________________ | From: George Huxtable | To: NavList@fer3.com | Sent: Sat, January 9, 2010 5:30:43 AM | Subject: [NavList] Re: QMOW Days work in Navigation | | Peter Hakel wrote- | | " ...experience has apparently shown that doing a running fix is preferable | to getting longitude from the time of LAN. This is very interesting. Does | anyone know how this rule was established? Has anyone tried both methods | and compared their accuracy? It is also conceivable that nobody has really | tested this and the reasons are historical: i.e. the LAN is used to give | latitude ONLY and thus the running fix is the next best thing to establish | position." | | ================================== | | Peter has it right. Although much attention has been given on this list to | the determination of longitude from observations around Local Apparent Noon, | mentions of its use at sea in actual chronometer-navigation are rare. That | is because its defects were recognised, as a method which would require | extended and precise observational data, to achieve a mediocre result; there | were better ways of doing the job. The moment of Local Apparent Noon was | indeed calculated, but that was to provide the best monent to take an | altitude of the Sun to determine latitude, which would (unlike the maximum | observed altitude) be unaffected by the vessels course and speed. | | [rest deleted by PH] | | | |