gistfile1.cs

public Double CalcDistanceOfTwoGpsCoordinates(Double latitudeA, Double longitudeA, Double latitudeB, Double longitudeB)  
{ 
      var pk = 180 / Math.PI; 
     
      var a1 = latitudeA / pk; 
      var a2 = longitudeA / pk; 
      var b1 = latitudeB / pk; 
      var b2 = longitudeB/ pk; 
     
      var t1 = Math.Cos(a1) * Math.Cos(a2) * Math.Cos(b1) * Math.Cos(b2); 
      var t2 = Math.Cos(a1) * Math.Sin(a2) * Math.Cos(b1) * Math.Sin(b2); 
      var t3 = Math.Sin(a1) * Math.Sin(b1); 
      var tt = Math.Acos(t1 + t2 + t3); 
       
      return 6366 * tt; 
}