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    Re: Happy Equinox Day
    From: Greg R_
    Date: 2007 Mar 22, 00:23 -0700
    > What were the odds of obtaining as good an
    > observation as the slope will produce with just the one sight?
     
    Good point, but I can usually manage to get the latitude (derived directly from peak Hs, of course) within a mile using the technique I mentioned earlier. Don't know if that has more to do with careful technique or just plain luck (or maybe a little of both), but I'm very careful about making sure I get a really good horizon "kiss" before I tell myself "Mark it".  
     
    I guess it comes down to just how well you trust yourself to get accurate sights with your particular sextant - today the sun was playing hide-and-seek behind some residual clouds after a weather system moved through (I was changing filters every few seconds either to be able to see it at all or to keep from frying my retina), so those two sights that I got at the peak were really all I had to work with.
     
    --
    GregR
     
     
     
    ----- Original Message -----
    From: "P F" <peter.fogg---.com>
    Sent: Wednesday, March 21, 2007 11:58 PM
    Subject: [NavList 2455] Re: Happy Equinox Day

    >
    > Bill:
    > > Excellent point from a practical standpoint IMHO.  You may recall a long
    > > series pd posts on averaging years ago where the academic viewpoint claimed
    > > that averaging should be avoided near LAN.
    >
    > Although this technique is similar to averaging it enables outliers
    > (statistics talk for, in our context, gross errors) to be eliminated,
    > thus an improvement on averaging that includes and is polluted by
    > them.
    >
    > Take another look at "An example of a slope", 10 March 07.
    >
    > 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 less 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
    > position.
    >
    >
    >

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