NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Plotting practice question
From: Rick Emerson
Date: 1999 May 16, 21:04 EDT
From: Rick Emerson
Date: 1999 May 16, 21:04 EDT
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