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Why does GPS show the Prime Meridian in the wrong place

Article by: Darren Griffin
Date: 20 Aug 2015

Sticking with the subject of maps and datums, a frequently asked question is why GPS does not show the Prime Meridian at Greenwich as zero? This issue also occurs in Google Maps, with both claiming the Meridian is about 100m to the east. But why?

The differences occur because when the longitude was first calculated, the Earth was assumed to be perfectly round whereas it's actually an oblate spheroid, which means it's fatter at the equator.

The error that resulted from this went unnoticed until the late 1960's when the first satellites were launched. A receiver was mounted on the roof of the Greenwich Observatory and calculations showed that an offset of 5.54" existed thanks to the more accurate calculations that were possible. This offset has since been incorporated into the WGS84 datum.

It's largely elementary now unless you're a tourist in Greenwhich park, or a guide at the observatory!

Source: Royal Observatory

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Posted by yealmgps on Fri Aug 21, 2015 7:31 am Reply with quote

I was at Greenwich a few months ago and asked the Guide about the offset in the Prime Meridian and he could not explain it!

Posted by pdfbt40 on Fri Aug 21, 2015 7:43 am Reply with quote

Its actually a bit more complex and political if I read the story correctly.
As more work was done in 1920s to calculate longitudinal references in America, the International Reference Datum was taken from Maryland Observatory, not Greenwich, naturally.
Now with greater GPS and topographical data, a back calculation from Maryland to Greenwich moves the 0 degree latitude from the defuncted meridian by ~100m

Posted by pillboxman on Fri Aug 21, 2015 10:31 am Reply with quote

OS False Origin is based near the Isles of Scilly, at 49 45" 58' N: 7 33" 23' W.

If the meridian is wrong how does this effect OS maps (if it does)?


Posted by challow on Fri Aug 21, 2015 10:41 am Reply with quote

But the best part must be, that the French are still very unhappy that London was chosen over Paris, in a world wide vote in 1864. I fact they were so pissed off that they stubbornly refused to vote and continued to use their meridian for a further 25 years! Eventually capitulating in 1909.

Then in 1994 they had 135 bronze medallions struck and placed in the pavements of Paris, marking their meridian and even as late as 2000 they were still at when they began planting a 600 mile long line of trees making the French meridian through the whole of France.

I suppose you have to admire their misplaced resolve/stupidity in continuing to push a meridian that no one uses anymore, including them Very Happy

Posted by pdfbt40 on Fri Aug 21, 2015 10:55 am Reply with quote

If its true, I'll see if I can find the article, isn't it also typical of the Americans to steal the International Datum just because their doing the further work and paying for it.

I suppose we should have sent in the gunboats and forced them to move the Datum by 100m to match OUR meridian.

Posted by pdfbt40 on Fri Aug 21, 2015 11:08 am Reply with quote


Posted by pillboxman on Fri Aug 21, 2015 11:28 am Reply with quote

Thanks pdfbt40

If I am reading that web page correctly it seems to suggest that it makes no actual difference as all the references are to a false zero anyway.


Posted by pdfbt40 on Fri Aug 21, 2015 12:20 pm Reply with quote

Well yes. As man has known from before Mercator, even to represent a perfect sphere on a flat piece of parchment as a map was bound to be inaccurate. An imperfect, knobbly ellipsoidal sphere is even worse, even if you swap to a 2D grid system. Hence individual datum points and grid systems.
But now we have 3D global data sets, it should be possible to have a single origin. Nationalism, as always, means we can't.

Posted by M8TJT on Fri Aug 21, 2015 12:44 pm Reply with quote

pdfbt40 Wrote:
I suppose we should have sent in the gunboats and forced them to ......
We did in 1775. And lost.

Posted by pdfbt40 on Fri Aug 21, 2015 4:57 pm Reply with quote

M8TJT Wrote:
pdfbt40 Wrote:
I suppose we should have sent in the gunboats and forced them to ......
We did in 1775. And lost.

But worked in mid-1800 when we protected our illegal drug running......

Posted by Lincslad on Fri Aug 21, 2015 5:18 pm Reply with quote

I am also fatter at the equator nowadays. Confused

TomTom XL V4 Europe

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