Calculate distance between 2 GPS coordinates

Question

How do I calculate distance between two GPS coordinates (using latitude and longitude)?

Answer 1

// Maybe a typo error ?
We have an unused variable dlon in GetDirection,
I assume

double y = Math.Sin(dlon) * Math.Cos(lat2);
// cannot use degrees in Cos ?

should be

double y = Math.Sin(dlon) * Math.Cos(dlat);