On Tue, Jun 29, 2004 at 09:29:48AM -0400, IvorW wrote:

More to the point is the accuracy of the other factor - the radius of the Earth.

The variation is of the order of 20km in 6370km.

If you assume that this is a constant, you are assuming that the Earth is a sphere.

The deviations in distance measured along the surface compared to that calculated from the "mean sphere" will, if my sums are correct, produce a maximum error of up to 1m in 2km at the poles and a shade more than that at the highest point on the equator. At the poles we'd calculate 2.001km, at Mount Kenya, about 1.999km.

That one metre only matters if you're trying to serve someone their soup by dropping it from orbit, not if you're merely trying to hit the right restaurant.

Even if my sums are wrong and I'm out by a factor of a HUNDRED, the maximum error is a mere 5%, or 100m in 2km, or 25m in 500m. Which still doesn't matter when you're trying to find a restaurant near the theatre.

Also, how would you go about measuring the distance from a given point on the Earth's crust to the centre of the earth?

The above shows that we can safely assume that the earth is a sphere with a radius of 6370km.