Why is the GRS80 ellipsoid used for GDA, instead of the WGS84 ellipsoid?
The two ellipsoids are effectively identical. When the WGS84 system was developed it was based on the GRS80 ellipsoid, but computational techniques resulted in a small difference in the flattening. When used to express earth-centred Cartesian positions (X, Y, Z) as latitude, longitude and ellipsoidal height, these two ellipsoids result in a difference of less than 1 millimetre.
