Greenwich meridian

[AntarcticPilot]

Yes, each country had its own lat/long model that suited their land mapping. This caused interesting problems when they were projected out in to the North Sea and defining boundaries in the early days of oil/gas exploration. A new North Sea geodetic datum was agreed for rig positioning etc. Then GPS came along and that was the first international geodetic datum. The Russian sat nav system used their own datum, but also being based on the earth's gravity is very close to WGS84. If @AntarcticPilot is around he can probably go into further detail.

Sorry for the late response! This post (GPS Spoofing) gives the low-down!

The global datums (WGS72, WGS84, ED50 and the like) all specify an ellipsoid of revolution and its orientation with respect to the solid earth. All modern datums in widespread use are indistinguishable. There are specialized datums used to measure continental drift and millimetre-accurate positions, but they don't interest us!

Just a run down - The best fit of a sphere to the Earth's surface deviates from the actual surface by about +/- 11km. It's perfectly adequate for celestial navigation; the errors arising from that assumption are way less than the errors in the method. An ellipsoidal datum such as WGS84 deviates from the actual surface (actually, the Geoid, a surface of equal gravitational force) by about +/- 100m.

The zero longitude of WGS84 is not set by a mark in the ground; it's a "best fit" of the positions of Very Long Baseline Interferometry observatories. So the zero is close to the brass strip at Greenwich, but a few hundred metres away. Those who set the brass strip in place would have regarded the difference as indistinguishable!

 

[lustyd]

I think I prefered the noddy explanation from the video. They essentially said that the GPS zero is exactly where they originally wanted it - directly under a given star at a given time (or rather the time was zeroed by that star being directly overhead). Since we kept the time the same the zero is where it is because they were measuring from the wrong place, but zero is still under that star as intended