There are many ways to calculate the distance between two points on the earth's surface, defined by their latitude and longitude. The methods vary in complexity and accuracy. Generally the simpler the method, the less accurate it is. Some possibilities are given below. This coincides with the circumference of a circle which passes through both points and the centre of the sphere.
Geodesic distance is the shortest distance between two points on a spheroid.
Normal section distance is formed by a plane on a spheroid containing a point at one end of the line and the normal of the point at the other end. For all practical purposes, the difference between a normal section and a geodesic distance is insignificant. This method assumes a spheroidal model for the earth with an average radius of 6364.963 km. It has been derived for use within Australia. The formula is estimated to have an accuracy of about 200 metres over 50 km, but may deteriorate with longer distances.