Calculate between two points on the earth

/*
Definitions:
South latitudes are negative, east longitudes are positive
Description:
This routine calculates the distance between two points (given the latitude/longitude of those points).
Passed to function:
@lat1, @lon1 = Latitude and Longitude of point 1 (in decimal degrees)
@lat2, @lon2 = Latitude and Longitude of point 2 (in decimal degrees)
@unit = the unit you desire for results
where: 'M' is statute miles
'K' is kilometers (default)
'N' is nautical miles
*/
CREATE FUNCTION dbo.fnGetDistance
(
@lat1 float,
@lon1 float,
@lat2 float,
@lon2 float,
@unit char(1)
)
RETURNS float
AS
BEGIN
DECLARE @theta FLOAT
DECLARE @dist FLOAT
DECLARE @pi FLOAT
SET @pi = 3.14159265358979323846
SET @theta = @lon1 - @lon2
SET @dist = sin(@lat1 * @pi/180) * sin(@lat2 * @pi/180)
+ cos(@lat1 * @pi/180) * cos(@lat2 * @pi/180) * cos(@theta * @pi/180)
SET @dist = acos(@dist)
SET @dist = @dist * 180/@pi
SET @dist = @dist * 60 * 1.1515
IF @unit = 'K'--Kilometer
SET @dist = @dist * 1.609344
ELSE IF @unit = 'N'--Nautical Miles
SET @dist = @dist * 0.8684

RETURN @dist
END