NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Plotting practice question
From: Tony S
Date: 1999 May 18, 00:59 EDT
From: Tony S
Date: 1999 May 18, 00:59 EDT
Rick: My question related to your own statement .... > > overall errors can be inferred from the direction and magnitude of the > > change, but it's not logged travel. ^^^^^^^^^^^^^^^^^ This was not clear to me inasmuch as we use "log" in several ways and for differing purposes. A true fix (not running fix) is the best we can come up with, not so? Confidence level can be quite variable since "our" sights are from a very unstable platform; compare to a ship, for instance. Dan: re your question ...will you buy 3 to 5 miles? ;) Tony Rick Emerson wrote: > > anthonys@XXX.XXX writes: > > I think we're both on the same wavelength. It is clear that a new track > > begins with a confident true fix. > > Er, practically speaking, the adjectives "confident true" are > sometimes loosely applied (on a day when nothing seems to work and all > the numbers thumb their digital noses at you [g]) but, per Bowditch, > yes. > > > What puzzles me is what you describe as "logged travel". Are you saying > > the ship's mechanical log, which might or might not be accurate, is > > significant? > > I'm not sure what question you're asking here. Let me take a few > guesses, though. > > 1) When restarting the track from a [new] fix, the log reading remains > undisturbed but is, of course, noted for future DR work. > > 2) While a log might be prone to error, it's wise to at least > determine the approximate order of magnitude (e.g., .1 nm in 10 nm, or > something like that) if not the actual error. If you don't have at > least a predictable log, there's little hope of accomplishing > meaningful navigation except by direct fixes using landmarks (remember > that celestial nav uses, in part, DR info). > > 3) The distance from the last DR (prior to the fix) to the fix is not > included in the ship's distance covered because, quite simply, the > ship didn't travel that leg. Very loosely, the true track was > something extended back from the new fix. > > [...] > > > Er, not quite, although you do have the basic points (e.g., DR error > > > is cumulative) down. I went back to Bowditch and in sections 703 and > > > 704 (pp 114-115) of the '95 edition it clearly says the track is > > > restarted with the fix. It also says, and this is the answer to the > > > question I raised, nothing about counting the distance between the > > > last DR point and the fix. As you suggest, information about the > > > overall errors can be inferred from the direction and magnitude of the > > > change, but it's not logged travel. > > Rick > S/V One With The Wind, Baba 35