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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Accuracy of sextant observations at sea (Mr David Burch's ARTICLE on "Averaging Celestial Sights")
From: Peter Fogg
Date: 2010 Dec 6, 09:56 +1100
From: Peter Fogg
Date: 2010 Dec 6, 09:56 +1100
Antoine, you wrote:
THE APPROACH M. BURCH IS USING IS NOT A LEAST SQUARE APPROACH.
Antoine, I'm not sure I'm following you here at all. An example of LEAST SQUARE APPROACH as applicable to nav could be choosing the fix position inside a triangle, to ensure the true centre of the shape is chosen. This could be wasted effort, since the observation position may not be in the centre of the triangle, or even within it, but at least the statistical 'least square approach' enables that derived fix point to be established quite precisely.
But here we are dealing with a series of successive sights of the same body taken over a short period, and ways of analysing them to resolve error. This could be to establish whether any outliers exist, although there are other advantages. The end result is a decision about the best altitude/time set to use for sight reduction, after consideration of the pattern of sights compared to the slope line of their apparent rise or fall over the period of observation.
Where does 'least squares' come into this? As best I can understand your post, you appear to be crying foul since greater weight has been accorded some sights than others.
If this is correct then I can only say that doing so is the whole point of the exercise. Rather than treating all points as equally valid, which averaging does, you provide yourself with a picture of your sights to assist you to choose which sights to favour over others, if any, and to what extent. Perhaps based on your recollection of taking one of those sights while the platform was moving more than during the other observations, for example. This is much of the beauty and appeal of this technique.
On the other hand, if I have completely misunderstood what you are getting at, perhaps you could try set me straight.
HIS CHOSEN APPROACH ESTABLISHES AN A PRIORI DE FACTO DISCRIMINATION ON SOME OF THE DATA, AND IT ALSO GIVES AN A PRIORI HIGHER IMPORTANCE TO OTHER DATA.
Yes. Well, not quite A PRIORI since you have to graph the sights first, then consider them, then decide which sights to favour over others, if any. If none, then the technique is similar to averaging.
NOT USING A LEAST SQUARE METHOD IS EQUIVALENT TO NOT TREATING ALL DATA EQUALLY JUST FROM THE ONSET.
Quite.
PS: Much as I would have loved to set this out in French its much easier, and especially quicker, to take advantage of all the effort you have gone to learn such excellent English. What a storm in a teacup! LAZINESS VINCIT OMNIA
However, I may make so bold as to post in French, or some other language, on another occasion.
