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    Re: errors in plotting and a possible/partial fix thereof, as menti...
    From: Peter Fogg
    Date: 2010 Dec 30, 10:50 +1100
     George Huxtable wrote;
    Initially quoting Gary -

    I am attaching a revised MOO table for your ship, 18 knots and a 5 minute
    period that you can use for advancing your LOPs and for figuring the slope
    method for eliminating random error.

    Then George opined:
    Were those words intentional? If he has evidence for any method of
    eliminating, or even reducing, random error, that does more than simple
    averaging of multiple observations, we would all be very interested.

    Nah.  For quite a long time I was somewhat bemused at how difficult otherwise skilled and interested navigators seemed to find this fairly simple idea and, for anyone willing to try it, the self-evident virtues of using the slope technique to analyse a series of sights in order to reduce random error. 

    Let's remember that at worst (ie; in cases where the method has little to offer since the pattern of sights shows little scatter) it tends to give a result similar to averaging, while at best (ie; where there are significant outliers) creating good data from bad, which averaging can't do, while in any case providing a picture of those sights to ponder, together with the actual slope of the apparent rise or fall of the body observed, to compare those sights against.

    Nevertheless, over time it seems to me that this situation has changed, and I get the impression that many of our regular posters understand and have tried the technique, use it at least occasionally and now accept its virtues.

    In other words George, many of those "very interested", if not all, have moved past you.

    However, I'm not so sure that the implications of having reduced random error as best can be practically done before plotting the triangle have fully sunk in.  The recent interest in the symmedian point is an example.  Apparently it is a better centre of the triangle, assuming the position lines sides of the triangle don't meet at a point due to random error.

    That's fine but could be rather irrelevant if the aim, once random error has been dealt with, then becomes to deal with non-random error.  Dividing the angles formed by intersecting position lines, then following those bisecting lines to where they meet at a point, this point being the reduction of that triangle back to a single point free of non-random error, does this.  If the azimuths have a greater spread than 180 degrees this point will lie at the centre of the triangle.  If less than 180d the meeting point will lie outside. 

    Always; no 25% here!  The whole 25% tale is a somewhat irrelevant furphy for anyone using these two simple tools, since the basis of the 25% chance of the fix lying within the triangle relies on an assumption that only random error is present, and I am still waiting for anyone, yourself included George, to support that assumption.

    Armed with these two simple tools the navigator is ready to deal with both random and non-random error in a practical manner which results in a fix, not free of both - that would be unrealistic; some error will probably remain - but most likely with a reduced extent of both.  A better fix.


       
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