A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Position lines, crossing
From: Henry Halboth
Date: 2006 Dec 11, 12:30 -0500
From: Henry Halboth
Date: 2006 Dec 11, 12:30 -0500
Hi Lu, Let's clarify one thing before going any further. This discussion has related, I think, to the intersection of lines of position obtained by celestial observation, not to the accuracy of the resultant position which may not be reflected by the nicety of intersection. One criterion sometimes applied is that of "repeatability" , i.e., the coincidence of multiple positions obtained in a static location over a period of time and obtained by the same means - some years ago, the USCG established experimental Loran stations for just this purpose, and the results seemed rather amazing as respects the divergence of the resultant positions. Although they have almost certainly been done, I know of no similar experiments with GPS, or for that matter any other form of position finding - potential accuracy would certainly be better served by such work, rather than by repeated quotations from texts, the authors of which may well have never have navigated a ship at sea and are published with the dollar market uppermost in mind. It should be remembered that, until relatively recent time, most charts were constructed on the basis of celestial observation, usually but not always utilizing the artificial horizon. Most of these charts, albeit not all, have been proved to be amazingly accurate - as a matter of simple fact, in the late 1940s, there being no other charts immediately available, I was obliged to use a 1908 BA blueback chart to navigate portions of the West African coast, and found no appreciable discrepancy of position anywhere. For years, my only gauges of position accuracy were that the ship remained always afloat, and the accuracy of any particular landfall. I have had perfectly wonderful looking fixes and, a few hours later made the intended landfall 6-miles or so either side of the intended course line while at other times it was dead on. On one occasion, it was predicted that we would raise Green Point Light at about 0600 - I was called at 2400 with Green Point and Slangkop clearly above the horizon and the 3rd Mate in a bloody panic; cross bearings quickly established that we were some 52-miles off Green Point and with plenty of time for a good snooze before arrival. It was the most significant case of refraction that I recall witnessing at sea. Any position established at sea by whatever means must be viewed with suspicion and the navigator must use every trick in his bag, the price of safety being eternal vigilance - and nothing could be truer as respects navigation. That being said, I simply refuse any static textbook definition of sight accuracy - such definitions, in my humble opinion, lead to sloppy if not careless workmanship and demean the potential of celestial navigation; generally they lead to excessive "rounding off" which in itself is conducive to accumulated errors. I have had good, bad, indifferent and wonderful results with celestial navigation and it was my job to be able to differentiate, based on existent conditions whatever they might be. Normally, I endeavored to work to the accuracy permitted by commercially available plotting sheets, and am of the opinion that average results, under good conditions of horizon and ship steadiness, were probably in the range of 2.5-miles of the truth with more often than not good LOP intersections. Let there, however, be no mistake, there were also bad results - just not too many. Regards, Henry ----- Original Message ----- From: "Lu Abel"
To: Sent: Monday, December 11, 2006 12:56 AM Subject: [NavList 1881] Re: Position lines, crossing > > With deepest respect, Henry, a previous poster pointed out that most > texts suggest that LOPs taken at sea are at best good to +/- 2 NM and > that for small craft +/- 5 NM is more likely. I agree. So I'm > wondering under what conditions your sights were taken and/or what scale > you used in plotting them! > > I don't mean in any way to demean your skill, I just am reflecting the > track of this discussion, which is that any navigator worth his salt > will not only be able to plot a LOP on a chart, but also fully > understand the uncertainties associated with it. > > Lu Abel > > halboth wrote: > > Hi Peter, > > > > I most certainly am Peter, and have hundreds, if not thousands, of > > positions in my navigation workbooks to demonstrate this fact. > > > > Regards, > > > > Henry > > > > ----- Original Message ----- > > *From:* Peter Fogg > > *To:* NavList@fer3.com > > *Sent:* Sunday, December 10, 2006 11:55 PM > > *Subject:* [NavList 1877] Re: Position lines, crossing > > > > Henry wrote: > > > > In my opinion, "cocked hats" are most often the result of > > altitude errors induced by varying horizon conditions due, in > > the case of stars, to failing light conditions. > > > > > > Henry, are you saying that given ideal conditions (as you have > > described) LOPs can in practice meet, or almost meet, at a common point? > > > > > > > > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavListemail@example.com -~----------~----~----~----~------~----~------~--~---