In uk.transport.london message
, Fri, 23 Jan 2009 20:57:06, Recliner
posted:
"David A Stocks" wrote in message

"Graculus" wrote in message
...
(No, GPS isn't accurate enough to say if its on the up fast/down
slow/etc track.)
Should be - I get accuracy down to about a metre on a very old
hand-held GPS box.

How can you be sure that it's so accurate? I took my Garmin wrist GPS
to Greenwich Observatory, and was crestfallen to discover that my GPS
wasn't nearly as accurate as it claimed. As I recall, it claimed to be
accurate to something like 15', but I had to move about 100' east from
the brass strip before it thought it was at logitude 0 degrees exactly.
I also have doubts about the vertical elevation it reports, though
that's not an issue for trains.

See
http://www.absoluteastronomy.com/topics/Royal_Observatory,_Greenwich
and http://en.wikipedia.org/wiki/Prime_Meridian.

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