I was at the Greenwich Old Observatory in London on Thursday, standing on the prime meridian with a GPS -

I was expecting it to be accurate to within the 9ms that the machine was telling me it was operating to.

But no - it was telling me I was still (roughly) 100m W....

I know the machine is accurate, as it matches up perfectly with Autoroute mapping software, to within +/- a few ms.

So what's the explanation?....

Well for a start, after a bit of research, it appears the line marked on the ground is the older 'Airy' line, whereas the one used on maps is apparently another one, the 'Bradley' line, which is 19ft off....but that doesn't account for the 100m error...

Any suggestions?

Dave L

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Edited 1 time(s). Last edit at 07/03/2005 06:27PM by Dave L.