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	my $lat0 = shift @_;
	my $lon1 = shift @_;
	my $lat1 = shift @_;
	
	# decimal to radian
	$lon0 = deg2rad($lon0);
	$lon1 = deg2rad($lon1);
	$lat0 = deg2rad($lat0);
	$lat1 = deg2rad($lat1);
	
	# Use the haversine formula for distance calculation
	# http://mathforum.org/library/drmath/view/51879.html
	my $dlon = $lon1 - $lon0;
	my $dlat = $lat1 - $lat0;
	my $a = (sin($dlat/2)) ** 2 + cos($lat0) * cos($lat1) * (sin($dlon/2)) ** 2;
	my $c = 2 * atan2(sqrt($a), sqrt(1-$a));
	my $distance = $c * 6366.71; # in kilometers

	return $distance;
}