Note: the accuracy quoted by Vincenty applies to the theoretical ellipsoid being used, which will differ (to varying degree) from the real earth geoid. If you happen to be located in Colorado, 2km above msl, distances will be 0.03% greater. In the UK, if you measure the distance from Land’s End to John O’ Groats using WGS-84, it will be 28m – 0.003% – greater than using the Airy ellipsoid, which provides a better fit for the UK.