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    Re: At the centre of time
    From: Douglas Denny
    Date: 2009 Oct 20, 16:45 -0700

    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 
    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.
    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
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