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I believe the reason is the different geodetic frames of reference used for 
the Earth's geoid between the Ordnance Survey of Britain and GPS system geoid 
used.

The Greenwich meridian is(was?) mapped by and linked to the Ordnance Survey to GB36 coordinates.

GPS however uses a different coordinate system for the Earth's geoid:- WSG84.

For example: my location by GPS WGS84 is 
Lat: N. 50-49.910 Long: W. 000-51.300 

using the same GPS unit, but set to GB36 coordinates it is:
Lat: N. 50-49.880  Long: W. 000-51,209

There is a map somewhere in the Journal of the Royal Institute of Navigation 
showing the discrepancies between BG36 and WGS84 for Britain.  It is greatest 
somewhere in the NW of Scotland if I remember rightly.  I'll post it if I can 
find it for interest's sake.
----------------

There is a very interesting story regarding the Greenwich observatory and 
mapping coordinate systems - from WW2.

The use of the highly accurate microwave bombing aid OBOE revealed an 
interesting anomaly in the relative mapping between the British Ordnance 
survey grid for maps used in Britain, and for those used on the continent 
based on the Paris observatory.

Because OBOE was so very accurate even at distances approaching the Rhur in 
Germany - down to a few yards - there was a requirement to ensure accurate 
bombing,  for very accurate assessment of the distances between the OBOE 
stations and the target for bombing.  Use was made of maps of the continent 
and those of Britain.

As part of this exercise,  a check was made of the link between the two 
coordinate systems which depended on the survey made (I think in the late 
nineteenth century {someone will be able to correct me on this no doubt} 
between London, Greenwich and the Paris observatories,  when it was found a 
discrepancy of about 17 feet was discovered in the exact position of the 
telescopes which made the observations for the establishing of the survey 
grids at London and Paris.
This was disconcerting because it had not been found before and was not 
acceptable for the survey accuracies involved.
No one knew how this came about ......

......... until someone suddenly realised that the transit instument at 
Greenwich used for the survey was the 'old' (Bradley, I think) instrument and 
not the 'new' Airey transit instrument currently used.

The Bradley instrument was 17 feet away in the other room - next to the Airey instrument !
Problem solved.

I know the last person who used the Airey Transit instrument on a regular 
basis as it was used until around the 1960's I believe. He is a very pleasant 
and most interesting chap to talk to. He is the chairman of an historical 
instruments society here in Britain.

Douglas Denny.
Chichester. England.

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Original text:

Interesting point regarding the shift of the prime meridian.  I encountered 
the same thing when I was there, my GPS did not agree with the stripe in the 
courtyard.  Aside from the political envy expressed by one reviewer as to why 
the prime meridian shifted, what were the reasons for that shift?

Best Regards
Brad





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