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    Re: Rejecting outliers
    From: Peter Hakel
    Date: 2011 Jan 7, 18:07 -0800



    From: George Huxtable <george{at}hux.me.uk>
    To: NavList@fer3.com
    Sent: Fri, January 7, 2011 4:56:42 PM
    Subject: [NavList] Re: Rejecting outliers

    I wish to return to Peter Hakel's posting of 3 Jan.

    I had written, on 2 Jan,

    "You seem to have ended up with a best-fit slope of about 24' in a 5-minute
    period, as I did when allowing Excel to make a best-fit, when giving it
    freedom to alter the slope as it thought fit. But the slope can be
    pre-assessed with sufficient accuracy from known information, and unless
    there is some error in the information given, such as an (unlikely) star
    mis-identification, we can be sure that the actual slope is nearer to 32',
    and the apparent lower figure is no more than a product of scatter in the
    data. This is a point that Peter Fogg keeps reiterating, perhaps the only
    valid point in all he has written on this topic."

    To which he replied-

    "Precomputing the slope requires DR information, which I chose to avoid
    using at the very beginning."

    =======

    But why do so? That's the whole point of this exercise, that in some cases
    the slope can be rather accurately pre-assessed, rather than derived from a
    rather poor set of data.

    ----------------------------------------------------------------------------------------

    Response from PH:

    In some cases yes, but perhaps not all.  Answering your question would require me to simply repeat what I had written earlier about bad weather, position tracking vs. position determining, getting a good quality data set later in better conditions, etc.  I also said that I have no problem with precomputing the slope; I was simply interested in developing a useful tool that deals with the problem of sight averaging from a slightly different starting point.  I cannot think of adding anything else that would explain my motivation better.  Given enough time, navigators can process their data by several methods and decide in the end with what result they feel most comfortable with.  Having a choice in that regard is not a bad thing.

    It seems to me, however, that your well-known skepticism is applied inconsistently in this case. On the one hand you object to rejecting data points that don't seem to belong (by common-sense visual inspection, like Peter Fogg does), while on the other you are OK with constraining data set to a predetermined trend (slope) by trusting the DR (common sense in SOME cases).  I thought your skepticism would lead you to join me in asking: "What if, in this case, the DR is not quite reliable?  What useful processing can we do with this data then?"  It appears that I was wrong on that account.


    Peter Hakel


    ---------------------------------------------------------------------------------------


    In this case, the altitude of the star had been measured to be around 66º
    30'. It's unlikely that the star had been misidentified, as Canopus is the
    brightest thing in that patch of sky, by quite a long way, its declination
    being 52º 42' S. The only other quantity that affects the slope is the
    observer's latitude, which I suggest we can take as well-known in this
    case, being right on the doorstep of the home-port of Sydney Harbour, at
    34ºS. Taken together, these give rise to a slope of around 32' over 5
    minutes.

    But Peter Hakel's computer assessment, when allowed free rein to choose its
    slope arrives at a slope of 24' over 5 minutes. To arrive at such a slope,
    the observation would have to be taken from a latitude of 31º 40' S, or
    thereabouts, being about 150 miles North of Sydney Harbour. That seems
    highly implausible.

    So let's have a look at the data that Peter Hakel's estimate was based on,
    in the attachment. I would agree that the better fit, with no other
    information to go on than those plotted point, would be the continuous line
    at a slope of 24. Peter's program says so, an Excel fit says so, and my eye
    says so. But is the dotted line, which is constrained to have a slope of
    32, in any way actually INCOMPATIBLE with those 9 data points, with all
    their scatter?  Not at all! If that dotted line represented the real truth,
    then those 9 points, would represent a perfectly plausible set of data,
    about that line.

    Asked to assess the slope, I would trust the observer's stated latitude,
    then, rather than the attempt to fit a slope to those widely-scattered data
    points. I suggest that Peter Hakel is allowing the computational
    mathematics to seduce him away from the commonsense of the real problem.

    Not that it matters, much. Because we have shown than the mean point (mean
    time, mean altitude) represents just as good a fit, whatever the assumed
    slope of the line.

    George.

    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

       
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