GetDistanceStraightLine.cs

public int GetDistanceStraightLine(double latOrg, double lngOrg, double latDest, double lngDest)
{
    const double radius = 6378.137;
    var dLat = latDest * Math.PI / 180 - latOrg * Math.PI / 180;
    var dLng = lngDest * Math.PI / 180 - lngOrg * Math.PI / 180;

    var a = Math.Sin(dLat / 2) *
            Math.Sin(dLat / 2) +
            Math.Cos(latOrg * Math.PI / 180) *
            Math.Cos(latDest * Math.PI / 180) *
            Math.Sin(dLng / 2) * 
            Math.Sin(dLng / 2);

    var c = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
    var d = radius * c * 1000;
    return (int) d;
}