So what is the real distance?
This reminds me of what someone told me 30 years ago. When working on the very first satellite navigation systems I found they calculated distances that were different to the longhand spherical trigonometry. - Navigation - the science of estimation and the art of guesswork. And here is the clue. What is the "real distance". A nautical mile is defined as a minute of arc, but because the earth is not a true sphere the length varies depending where it is measured. It differs by nearly 20m between poles and equator, so an average length is used. Of course, the satellites follow orbits that follow the shape of the earth - an oblate spheroid. So which is correct? the distance measured by GPS calculation from a satellite with a non-spherical orbit, or a land-based calculation with all sorts of averaging and compensation to arrive at a standard datum? Its easy to fix a position by lat and long.It's not quite so easy to decide exactly what the surface distance is between those two points.