Greenwich Prime Meridian?

John, I have read once that it is true; after the last topographic survey done in England, it appears that the meridian line marked in the Royal Observatory is not accurate. But I doubt it is as much as 100 meters.

One thing, though, is that there are several different chart datum. Since the introduction of the GPS, the WGS84 standard is in use and I think that is what Google Earth uses.

Furthermore, I know that on maritime charts for Norway, my country, while the relatives positions are very accurate (this island, relative to that one) the actual reference WGS84 position can be off by perhaps 100 or 200 meters. This is why yachtsmen are advised to be careful when navigating by GPS, plotting it on an old paper chart. New digital plotters are, of course, of better quality and properly adjusted.

But for me who started navigating in the 1960 with a sextant, the GPS is just fantastic.

Anyway, since you live near the Royal Observatory (which I always wanted to visit!) you may ask the people there about it and ... send me a private email; I am very curious of their saying.

The history of navigation, finding longitude, the maritime chronometer from John Harrison and the history of the Observatory is simply fascinating.

I've looked up known locations on Google Earth and there are times when what they show isn't exactly the location but very close.

no ure not wrong ure right :D