Apparently if you stand at Greenwich then your GPS will not say you are at longitude zero. This is because the GPS reference meridian is about 100 metres to the east of the prime meridian.
My question is: Why is the GPS Reference Meridian 100m to the East of the Prime Meridian?
Answer
I've answered this question in passing while answering another of your question.
The Greenwich Observatory was defined as a prime meridian, based on the observations by the astronomer Sir George Airy in 1851. London was selected as the official prime meridian for international maps by the International Meridian Conference in 1884.
When you use a GPS, by default it uses the WGS84 datum. This is different from the old coordinate system. WGS84 is a global datum based on calculations from readings across the earth, and its prime meridian is different. The difference is about 100 meters at Greenwich.
More details are available in these links:
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