the author of this app says:
Distance computes the geodesic distance between two points on the surface of the Earth to an accuracy of about 50 meters.
To get this degree of accuracy, the algorithm takes into account the spheroidal flattening of the Earth, rather than assuming that the Earth is a perfect sphere.
Enter the latitude and longitude for two points on Earth and tap the calculate button to get the distance in kilometres, miles and nautical miles.
Tapping the clear button erases all the values so you can enter new coordinates.