I'm sure Mike meant that the Greenwich Meridian Line is about 5" west, not 5' west, of where it is shown on Google Earth. (I just looked to confirm this!)
This discrepancy is simply due to Google Earth's adopted geodetic datum, WGS84. GPS receivers can often be set to show coordinates for other datums (data?) as well (for example, GB36, the Ordnance Survey of Great Britain 1936, on which the Greenwich meridian line is *much* closer to 0 longitude).
The datum choice probably explains the equator discrepancy in Ecuador as well. It's simply that each country, historically, conducted geodetic surveys that are internally consistent to high accuracy. But they don't join up perfectly with those done in other countries. The WGS84 datum is intended to apply to the entire world. We had a brief discussion of this issue in Sky & Telescope for August 2005, page 110.
I learned this lesson the hard way. When I went to find the gravestone of telescope maker Russell W. Porter in Port Clyde, Maine, I had GPS coordinates from a friend. After an hour and a half, no luck -- so I gave up. Only later did I learn that his receiver was set to WGS84 while mine was set to NAD27.
-- Roger
At 08:27 AM 7/23/2006 +0100, Mike Shaw wrote:
>>>>
Bill G. said: <<There is a large monument in Ecuador that marks the exact location of the equator, so they claim. Google Earth puts it about 780 feet south of the equator. Any thoughts on who is more accurate: Google Earth or the Ecuadorian surveyors? >> I have also noticed that, according the Google Earth, the meridian line at the Greenwich Observatory (surely correct) is at 5.26 minutes West. Mike Shaw 53.37N
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