Google Earth Community: Greenwich Longitude off by 106m in Keyhole?

Keyhole, Google Earth, and the GPS constellation (therefore car navigation systems, handheld GPS units, and even cruise missiles) use the WGS-84 World Geodetic System. The Flamsteed Astronomy Society has explained the implications of this quite nicely:

It is true that a GPS receiver held over the Meridian Line at Greenwich will NOT read longitude (0,0). The WGS84 prime meridian is in fact 102.5 meters to the east of the 1884 Prime Meridian at Greenwich. WGS84 stands for "World Geodetic System 1984" and is used by today’s GPS satellite Global Positioning System.

The World’s Prime Meridian marked at Greenwich is the “Airy Meridian” which is aligned to the eye-piece of George Airy’s Transit Circle that had been in use since 1851. It was adopted by the international community in 1884 at a conference in Washington DC. It is no less than the fourth meridian marked at Greenwich. The other meridians are all to the west, marching down the Meridian Building, and were established by Flamsteed, Halley, and Bradley. It was therefore something of a tradition for Astronomers Royal to build new meridian walls (which they did to be able to continue observations while installing new instruments. The geography of the Observatory hill and courtyard really forced them progressively eastward).

There was no “natural” or scientific reason for the Prime Meridian to be at Greenwich. It could have been placed at any location where a world-class observatory could refine position measurements. Agreement for Greenwich came from realisation that 70% of the world’s shipping was using Greenwich charts. (Mainly thanks to Maskelyne and the Nautical Almanac. After Harrison, chronometers had remained too expensive to become widespread very quickly). It didn’t hurt that the US Railroads had effectively standardised on the Greenwich meridian in 1883 when they adopted Dowd’s time zone system for their standard timetables. At the 1884 conference the delegate from France agreed not to oppose the adoption of Greenwich on the condition that Britain adopt the metric system. This the Government of Britain promised to do in 1884, and firmly plans to keep that promise when the time is right.

After 1884 all was (more or less) harmony until 1954 when positional astronomy in Britain moved from Greenwich to Herstmonceux and the Airy Transit Circle was finally retired. By the 1950s international timekeeping was being regulated by the BIH (Bureau International de l’Heure). The world standard was then based on an average of observations from several observatories and the average reference meridian had probably already wandered around 8 meters from Airy. The move to Herstmonceux required ‘recalibration’ to be done and this added another 10 to 20 meters. Continental drift is also responsible for a bit of movement since 1884.

With the advent of the GPS satellite system in the 1980s, accurate navigation could be done by any child using a hand-held computer with more compute power than existed in the entire world in 1954, and with access to 25 satellites in Earth orbit, each carrying two caesium atomic clocks. The system calculates position by receiving and comparing time signals from any three GPS satellites. It needs an internal “map” of the world in the form of a computer program. It wasn’t easy to create the computer map because the Earth isn’t a simple sphere. It has a complicated shape which required a technique called ‘best fit’ to develop the map based on an Earth geodetic model -- WGS84. This doesn’t fit the Earth’s surface exactly everywhere but juggles the map shape to find the position where it fits best at the most places it can. Try as they might, the best fit they could get at Greenwich put the WGS84 meridian to the east of Airy by a tad under 102.5 meters.

--------------------
Be seeing you,
Seer

Post Extras