I believe you are computing the length of arc on the surface of a sphere - which is cool... But the earth is not a perfect sphere - it's an "oblate spheroid". (it bulges around the equator and is not perfectly symetric) There is an algorithm used in the aerospace industry to compute the distance between two points on the earths surface. Do a google search on "Sedonos Equations". It gets very complicated... Some aerospace companies have patented their implementations of it.

SMB