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Re: Sextant Positions versus Map Datums?
From: Trevor Kenchington
Date: 2002 Jan 16, 8:07 PM
From: Trevor Kenchington
Date: 2002 Jan 16, 8:07 PM
Peter Smith wrote: > Between datums, yes. The "horizontal difference" I was referring to > was between the ellipsoids that underlie the various datums. When you > project celestial LOP's onto either a sphere or one of these > ellipsoids, you get pretty much the same result. It's the landmarks > that appear to move when you compare charts built on different > datums I must be missing something here. As I understand it, the various datums are different ellipsoids, each of which is a mathematically-perfect surface which closely (but not perfectly) matches the shape of the real globe, the early ellipsoids differing in which part of the real globe they most closely matched. Surely then it is not the landmarks which move (they, after all, are rooted on bedrock) but the positions of latitude and longitude lines which are defined relative to the particular ellipsoid? If so, Jared's question seems a valid one. For which datum (i.e. which ellipsoid) are the tabulated numbers in the various sight-reduction tables calculated? Or are are they in fact (as I rather suspect) actually calculated on an assumption that the Earth is a perfect sphere? [Most of the numbers in the Almanac presumably relate the positions of celestial bodies to the centre of the Earth and so are not affected by the chosen ellipsoid. Or am I off-base on that?] Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus