CREATE FUNCTION [dbo].[DistanceBetween]
   (
       @Lat1 AS REAL,
       @Long1 AS REAL,
       @Lat2 AS REAL,
       @Long2 AS REAL
   )
RETURNS REAL

AS

-- returns distance as the crow flies (in km) between two lat/long coordinates

BEGIN

DECLARE
       @dLat1InRad AS FLOAT(53),
       @dLong1InRad AS FLOAT(53),
       @dLat2InRad AS FLOAT(53),
       @dLong2InRad AS FLOAT(53),
       @dLongitude AS FLOAT(53),
       @dLatitude AS FLOAT(53),
       @a AS FLOAT(53),
       @c AS REAL,
       @kEarthRadius AS REAL,
       @dDistance AS REAL


SET @dLat1InRad = @Lat1 * (PI()/180.0)
SET @dLong1InRad = @Long1 * (PI()/180.0)
SET @dLat2InRad = @Lat2 * (PI()/180.0)
SET @dLong2InRad = @Long2 * (PI()/180.0)
SET @dLongitude = @dLong2InRad - @dLong1InRad
SET @dLatitude = @dLat2InRad - @dLat1InRad

/* Intermediate result a. */
SET @a = SQUARE (SIN (@dLatitude / 2.0)) + COS (@dLat1InRad) * COS (@dLat2InRad) * SQUARE(SIN (@dLongitude / 2.0))

/* Intermediate result c (great circle distance in Radians). */
SET @c = 2.0 * ATN2 (SQRT (@a), SQRT (1.0 - @a))

/* SET kEarthRadius = 3956.0 miles */
SET @kEarthRadius = 6376.5        

/* kms */
SET @dDistance = @kEarthRadius * @c

RETURN (@dDistance)

END