NavList:

Scott,
Did you visit the NMM web site linked by Richard Langley? It explains many of the issues involved. It ends with a curious sentence: "So all of these contributions may explain why the longitude of Greenwich in the Doppler systems would be the order of 100 m away from zero. "

MAY explain?? A bit waffly there!

The main issue is surely the one they discuss at length: different map datums. These were originally set up for the convenience of a given region.

Incidentally, all lines of latitude and longitude are similarly difficult to define at the level of 100 meters of less, not just the Greenwich meridian. There is a degree of arbitrariness since the Earth is by no means a sphere and the geoid is not a simple ellipsoid. In Ecuador, there is a monument marking the equator. If I remember correctly, it's called "Mitad del Mundo" (not to be confused with "Middle Earth"). It's not exactly on the equator, as tourists discover with their GPS-equipped devices. But not to worry, just down the street there's a "museum" (read, "tourist trap") where tourists are told that they are now standing on the "real equator" and they can prove it by letting water drain from a basin, first right at the equator, then just north of the equator, and finally just south of the equator. It's the Coriolis effect, ya see... And you thought science was only found in books! :) Naturally, the Coriolis effect has nothing to do with any of this, the equator is not involved, and in a basin a few feet across this wouldn't even work (without exceedingly careful preparations) thousands of miles from the equator. It's a simple sleight of hand trick: fill the basin with the flow crossing the drain right at the equator, or fill the basin with the flow slightly to the left/right of the drain south/north of the equator. The water can then be made to drain out clockwise/counter-clockwise on demand! They do a bunch of these silly games and also claim that the indigenous Ecuadorians knew how to find the "true equator" using the water trick. And did you enjoy the show?? Tips are all you owe. I also remember seeing the same tricks performed for Michael Palin and other gullible tourists on the equator in Africa on one of his travelogue shows many years ago.

-FER

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