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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?

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2 Answers 2

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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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a hundred meters in londen, equator or at the pole? –  ratchet freak Mar 4 at 11:54
    
@ratchetfreak: At London. –  Devdatta Tengshe Mar 4 at 12:16

The short answer is: there was a historical shift between the first global datum and the Greenwich meridian; and it continues to move (slowly) because of continental drift. You can find more detailed information on this site

In the late 1950s (under the auspices of the US Navy), the Applied Physics Laboratory (APL) of the Johns Hopkins University began the development of what was to become the world's first operational satellite navigation system. Known as Transit, it worked by making use of the Doppler effect, the same effect that makes a siren carried by a moving vehicle change in pitch as it passes. The surveyed longitude of the Laboratory's site in Maryland, as measured in the North American Datum (NAD27), became its assumed longitude in the first World Datum, the APL datum. It was this pragmatic adoption of the longitude coordinate on one ellipsoid as the assumed value on another that has caused the apparent shift not only in the position of the Meridian, but also of all other locations.

The size of the shift remained unknown until the summer of 1969, when an opportunity arose to measure it. A satellite receiver was set up on a platform above the roof over the Airy Transit Circle at Greenwich. The results showed that fixes resulting from the use of the satellite navigation system should have their longitude values shifted by 5.64" if the Greenwich (Geodetic) Meridian was to have its longitude as zero in this system. Although an academic paper on this subject was published in 1971, it appears to have been largely forgotten about until the mid noughties. The offset (since refined) also applies to the WGS84 datum used by current GPS systems. WGS84 was adopted as the global standard for air navigation on 1 January 1998 and soon afterwards by hydrographers for use on electronic and nautical charts.

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I believe Continental shift has nothing to do with it. The WGS84 prime meridian and the Greenwich meridian are just two different things. –  Devdatta Tengshe Mar 4 at 11:18
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What I say is that there was a historical shift of the prime meridian (this explains the 100 m), but this is still moving ( In the UK WGS84 latitudes and longitudes are changing at about 2.5 cm per year). I've tried to clarify. –  radouxju Mar 4 at 11:27
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+1 From Airy's time until now some places on the earth likely have moved as much as 150 meters relative to the Greenwich observatory (because some plates are drifting as much as 10+ cm/yr relative to others). Although (as @Devdatta objects) this fact about plate tectonics does not address the question it certainly puts the 100m shift into perspective--clearly indicating plate motion has something to do with the shift--and shows why a modern high-accuracy datum must have an epoch. –  whuber Mar 4 at 20:57
    
(2014 - 1884) yrs * 0.025 m/yr = 3.25 m –  martin f Mar 4 at 23:49
    
(2014 - 1884) yrs * 0.01 m/yr = 13.0 m –  martin f Mar 4 at 23:50

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