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    Re: Plotting practice question
    From: Tony S
    Date: 1999 May 18, 00:59 EDT

    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?  ;)
    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

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