apply formulae
double a = Math . Pow ( Math . Sin ( dLat / 2 ) , 2 ) + Math . Pow ( Math . Sin ( dLon / 2 ) , 2 ) * Math . Cos ( lat1 ) * Math . Cos ( lat2 ) ; double rad = 6371 ; double c = 2 * Math . Asin ( Math . Sqrt ( a ) ) ; return rad * c ; }