sub CalculateDistance($$$$)
{
 my ($Lat1, $Lon1, $Lat2, $Lon2) = @_;
 my ($nDLat, $nDLon);
 my ($nA, $nC, $nD);
 
 $nDLat = ($Lat1 - $Lat2) * 0.017453293;
 $nDLon = ($Lon1 - $Lon2) * 0.017453293;
 
 $Lat1 = ($Lat1) * 0.017453293;
 $Lat2 = ($Lat2) * 0.017453293;
 
 $nA = (sin($nDLat/2) ** 2) + cos($Lat1) * cos($Lat2) * ( sin($nDLon/2) ** 2 );
 $nC = 2 * atan2( sqrt($nA), sqrt( 1 - $nA )); 
 $nD = 6372.797 * $nC;
 
 return $nD;
}