NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Taking four stars for checking accuracy of fix - and "Cocked Hats"
From: R B Emerson
Date: 2008 Aug 05, 08:22 -0400
From: R B Emerson
Date: 2008 Aug 05, 08:22 -0400
Two thoughts... first, the amount of error is useful in trying to ascertain what went wrong. If the error is a "magic number" (that is, off by enough to suggest a transposition in digits or error of a whole minute or degree), at least the cause is reasonably certain. It's the slight errors that are really annoying. Maybe the boat moved, the body wasn't really on the horizon, the sextant was off the vertical, a second or two was lost in recoding the time... the list of reasons for these small but inescapable errors can be depressingly long. Second, I'm very reluctant to simply adjust the sight by a minute or degree and add it to the round of sites; there is no assurance the cause was as suggested, there's only a "good guess" as to the cause. It helps to remember that "Assume is the mother of all ****ups". Assuming that the error was "only" due to writing 45-23 for an altitude instead of 44-23 or 16:37:43 for 16:31:43 may work. And it may not. Better to shoot the body several times and discard the outlier altogether(*). In a sense, a fix should be something of a (pleasant) surprise. Certainly the navigator has a sense of where the boat is, the purpose of the sights is to determine the boat's location, not confirm the navigator's assumptions. After all, some assumptions can be made in error... Rick Emerson S/V One With The Wind (*) Agreed that if I were in a situation with no sights or only one or two and one of them seemed to suffer from a recording error, I'd be very reluctant to discard it. But I'd also have low confidence in the resulting fix or LOP. Bill wrote: > Peter wrote > > >> This has actually happened to me, on more than one occasion. While >> concentrating on the seconds, the wrong minute of time gets recorded. >> Or while focused on the minutes of arc displayed by the sextant, the >> wrong degree is marked (blame the scribe). >> >> This is also another example of why it is important to THINK about >> what those data points mean, rather than just feeding them into some >> blind number-crunching mathematical process. Once plotted its obvious >> that the point is an outlier, an apparent gross error. But before >> discarding the point its worthwhile checking that it is not just a >> whole minute of time out, or a whole degree. >> > > Excellent points, although it has never happened to me.> > Seriously, a major outlier often turns out to be exactly as you describe, a > minute of time or a degree of arc off. Embarrassing, but it always feel good > to *see* the relationship, ask, "What if?" and recover the data. > > Bill B > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---