It’s nice to see that Google Earth draws and measures lines as great circles – i.e. lines follow the surface of the globe – or geodetically speaking, the spheroid. It’s fairly obvious, really, given the 3-D nature of GE, but it’s not something you take for granted in a GIS, where measurements are generally made on a projected plane of some sort, and the result depends on the projection used.
As a sort of confirmation of this, I measured the distance from the north pole to the equator, the orange line here:
The reported result was 10,001,959 metres. Multiply this by 4 to get a circumference of the earth (a minimum circumference, given the flattened, oblate nature of the spheroid), and you get 40,007,836 m.
Look up the ‘textbook’ value of this circumference, calculated from the equatorial radius and the flattening factor, and you get….. 40,007,863 m.
So, it does indeed look as though Google Earth (a) measures on the spheroid surface and (b) it uses WGS84. all of which is perfectly well known anyway, but it’s nice to confirm it….