/// <summary>
/// The distance between two geographical coordinates with altitude.
/// </summary>
/// <param name="x1">Longitude 1</param>
/// <param name="y1">Latitude 1</param>
/// <param name="a1">Altutude 1 (km)</param>
/// <param name="x2">Longitude 2</param>
/// <param name="y2">Latitude 2</param>
/// <param name="a2">Altitude 2 (km)</param>
/// <param name="dx">Distance longitude (km)</param>
/// <param name="dy">Distance latitude (km)</param>
public static void DistanceBetweenPoints(double x1, double y1, float a1, double x2, double y2, float a2, out double dx, out double dy)
{
    double r = 6371 + (a1 + a2) / 2;
    double Deg2Rad = Math.PI / 180;
    double scfY = Math.Sin(y1 * Deg2Rad);
    double sctY = Math.Sin(y2 * Deg2Rad);
    double ccfY = Math.Cos(y1 * Deg2Rad);
    double cctY = Math.Cos(y2 * Deg2Rad);
    double cX = Math.Cos((x1 - x2) * Deg2Rad);
    double sizeX1 = Math.Abs(r * Math.Acos(scfY * scfY + ccfY * ccfY * cX));
    double sizeX2 = Math.Abs(r * Math.Acos(sctY * sctY + cctY * cctY * cX));
    dx = (sizeX1 + sizeX2) / 2.0;
    dy = r * Math.Acos(scfY * sctY + ccfY * cctY);
    if (double.IsNaN(dy)) dy = 0;
}