Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: On the integration of location and data
    From: George Huxtable
    Date: 2009 Nov 9, 13:59 -0000

    How splendid! Another top-grade, mind-blown rant from Frank Reed. We haven't
    had a really good one from him for some time, but this is one of his best.
    Replete with personal abuse, wild exaggeration, capital letters, full of
    sound and fury. His toes must have been firmly trodden on. It's given rise
    to some chuckles here, and no doubt in many households where Navlist is
    I do hope we aren't destined to relive the whole sequence of postings that
    arose in the thread "Learn the stars, by phone", back in May and June.
    Anyone interested can go back into that history, as I suggested. It will
    provide balance to some of Frank's wilder claims.
    Such devices, as with magnetic compasses, are only as accurate as the
    direction of the local Earth's field, with any perturbations, can be known.
    When the snags of using Earth magnetism as an azimuth reference were pointed
    out, Frank pointed to the precision claimed for the Skyscout, as 0.5
    degrees. When that claim was questioned as implausibe, he replied, in
    "That half-degree claim comes from the official specs. It's a believable
    claim, based on performance, but the exact level of accuracy (whether it's
    0.5 or 0.75 or 1.0 degrees) is not critical in any way to the device's use."
    Well, I might have gone along with that, if indeed 1.0 degrees was the
    limit. But when it transpired that the tolerance had been relaxed to 3
    degrees, without affecting Frank's enthusiasm, one wonders just how much
    imprecision Frank is prepared to accept. I hope he will tell us.
    Frank's response is this- "And just a reminder, George, you have still not
    even bothered to define "pointing precision". How strange to worry so much
    over something that you have not defined..." Nor, I might add, did Frank
    define it (in which he took refuge later). Nor did Celestron. All we can say
    is that however it was defined, it was later degraded to being 6 times worse
    than those initial claims, as relayed by Frank.
    However, none of this mattered, it was claimed, because such a device was
    capable of detecting the presence of local perturbation, and warning the
    user. I doubted whether the device would be able to detect sufficiently
    small perturbations, suggesting-
    "It could, perhaps, by detecting any significant difference in the total
    field strength, or the dip angle, from the value predicted for that location
    on the Earth. But could it establish, by such means, a deviation that
    changes the magnetic direction by 1degree? I don't believe it!"
    to which Frank responded- "I agree -- that wouldn't work."
    So, in its place, he speculated- "...measure derivatives of the field. The
    terrestrial field is quite uniform over considerable distances."
    The recent observations I reported, with a small bar magnet, demolish that
    notion. We're left with checking of the observed field against its predicted
    total value, and dip angle. Which, as Frank concedes, "wouldn't work". It
    doesn't. Not well enough.
    Frank recently pointed us to his posting in May, without alluding to the
    errors it contained. I reminded readers of those shortcomings, which seems
    to have triggered this explosion.
    I wrote- "In the absence of any magnetic perturbation, no doubt that
    instrument, or some sort of fancy-phone that works the same way, might do a
    reasonable job in pointing precision"
    Which drew this response-
    "Fine. That's all that's required. Let me repeat that a little louder for
    But is it? How, in this World, where iron and steel are everywhere, can the
    user know that there is no local magnetic perturbation? Clearly, the
    warning-indication isn't trustworthy.
    The defects in such a device are not in the device itself, but in the
    magnetic field it's trying to sense to determine azimuth.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList+unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site