NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Happy Equinox Day
From: Greg R_
Date: 2007 Mar 22, 00:23 -0700
>
> 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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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?
> 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@gmail.com>
To: <NavList@fer3.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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To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
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