My first custom function--the haversine formula

Hi
 
my first post and a chance to say how useful i have found this site and the linked videos 
 
I have recently been tasked with testing how far customers laddresses are from the business. I found a data base which had the longitude and latitude of every UK postcode,  armed with that information i came up with this custom function
 
Option ExplicitFunction getDistance(latitude1 As Double,longitude1 As Double,latitude2 As Double,longitude2 As Double) As DoubleDim earth_radius As DoubleDim Pi As DoubleDim deg2rad As Double
 
Dim dLat As DoubleDim dLon As DoubleDim a As DoubleDim c As DoubleDim d As Double
 
earth_radius = 6371Pi = 3.14159265deg2rad = Pi / 180
 
dLat = deg2rad * (latitude2 - latitude1)dLon = deg2rad * (longitude2 - longitude1)
 
a = Sin(dLat / 2) * Sin(dLat / 2) + Cos(deg2rad * latitude1) * Cos(deg2rad * latitude2) * Sin(dLon / 2) * Sin(dLon / 2)c = 2 * Sqr(Atn(a / Sqr(1 - (a*a))))
 
d = earth_radius * c
 
getdistance = d
 
End Function
 
I couldn't have done with out the help of the Youtube videos on custom functioms and a little help with the  trigonometery  from Google