def os_cart(inref):
''' Return a tuple of two 7 digit OS numeric grid refs from an XXNNNN type reference.
Also returns the accuracy of the original OS ref since this is lost when
the data are padded to two 6 digit grid refs.
Example: os_cart("SN109112") returns (210900, 211200, 100)
'''
#print inref
'''get numeric values of letter references, mapping A->0, B->1, C->2, etc:'''
inref = inref.replace(" ","") #Strip all spaces out
# Deal with the letters
l1 = ord(inref[0].upper())-ord("A")
l2 = ord(inref[1].upper())-ord("A")
# shuffle down letters after 'I' since 'I' is not used in grid:
if l1 > 7: l1 -= 1
if l2 > 7: l2 -= 1

e = str(((l1-2)%5)*5 + (l2%5)) #easting
n = str(int((19-math.floor(l1/5)*5) - math.floor(l2/5))) #northing

# Now the numbers
gridref = inref[2:]
e += gridref[:int(len(gridref)/2)]
n += gridref[int(len(gridref)/2):]
# Pad short refs with correct number of zeros for postGIS
# This does imply a greater accuracy then the original data suggest
# however and should be used with caution, see next line.
a = 1 * int(math.pow(10,6-len(n))) # Calculate the accuracy of original coordinates in metres.
n = int(math.pow(10,6-len(n))) * int(n)
e = int(math.pow(10,6-len(e))) * int(e)
return int(e), int(n), int(a)