Those helpful folks at the
UK Ordnance Survey have provided a fine reference
Guide to Coordinate Systems in Great Britain. It's well worth reading if you're interested in the intricacies of
coordinate systems,
datums,
surveying and
GPS. It's a bit technical, but if you can handle coding in Javascript you can probably get something out of it...
An interesting extract gives the general flavour:
- The WGS84 Cartesian axes and ellipsoid are geocentric; that is, their origin is the centre of mass of the whole Earth including oceans and atmosphere.
- The scale of the axes is that of the local Earth frame, in the sense of the relativistic theory of gravitation.
- Their orientation (that is, the directions of the axes, and hence the orientation of the ellipsoid equator and prime meridian of zero longitude) coincided with the equator and prime meridian of the Bureau Internationale de l’Heure at the moment in time 1984.0 (that is, midnight on New Year’s Eve 1983).
- Since 1984.0, the orientation of the axes and ellipsoid has changed such that the average motion of the crustal plates relative to the ellipsoid is zero. This ensures that the Z-axis of the WGS84 datum coincides with the International Reference Pole, and that the prime meridian of the ellipsoid (that is, the plane containing the Z and X Cartesian axes) coincides with the International Reference Meridian.
- The shape and size of the WGS84 biaxial ellipsoid is defined by the semi-major axis length a 6378137.0 metres, and the reciprocal of flattening 1/f 298.257223563. This ellipsoid is the same shape and size as the GRS80 ellipsoid.
- Conventional values are also adopted for the standard angular velocity of the Earth, and for the Earth gravitational constant. The first is needed for time measurement, and the second to define the scale of the system in a relativistic sense.
Now then, if you find yourself getting lost on the way from Foyle's to the nearest Lyon's tea shop, you can't say you haven't been warned!
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