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    A 'real-life' example of slope
    From: Peter Fogg
    Date: 2010 Dec 13, 18:49 +1100
    The attached file, an example of slope in action, comes from a post I made on 10 March 2007 [NavList 2278].  It may seem like a poor round of sights compared with Antoine's, but remember the crucial difference often ignored by our armchair navigators: the relative stability of the platform used.  Unless the sea conditions are abnormally calm, the near-perfection of Antoine's sights is in practice unachievable from the deck of a smallish sailing boat, in my experience.

    The analysis below comes from [
    NavList 2455] of 22 March 2007:

    Sights 1 and 3 have been discarded, as they cannot be matched to the
    slope. This slope, a fact, is then best matched to the pattern of the
    other sights that exhibit random error.

    What is the alternative to this technique? In this example, taking
    just the one sight could have been equivalent to choosing any one of
    these sights at random. What were the odds of obtaining as good an
    observation as the slope will produce with just the one sight?

    Of a poor sight (#1&3): 2 out of 9; 22%
    Of a mediocre sight (#2,4,6,7,8,9): 6 out of 9; 67%
    Of poor or mediocre: 8 out of 9; 89%
    Of an excellent sight (#5): 1 out of 9; 11%

    So at the cost of a little extra calculation and the drawing up of a
    simple graph this 11% chance has been converted to a 100% chance of a
    similar result to what appears to be the best sight of the bunch,
    together with all the other advantages of KNOWING a lot more about
    this round of sights, and being able to derive extra information (eg;
    standard deviation) at will.

    Is this a typical example? No. Typically there are fewer sights in the
    5 minutes, and NONE of the individual sights is as good as the derived
    slope; confirmed by comparing the resulting position lines to a known


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