Buckinghamshire CC ANPR cameras

[John Wilson]

Rather to my surprise Bucks CC have given me the details of the
hashing scheme used by ANPR cameras which implement the UTMC protocol
(which is, I think, all of the civil and police ANPR cameras). This
was the result of an FoI request.

D 0 Q are replaced with O (Q isn't used in the current numbering scheme)
1 is replaced with I            (I isn't used in the current numbering scheme)
5 is replaced with S
Y is replaced with V
8 and B are replaced with 3  (this may cause problems after 2030)
Z is replaced with 2
F is replaced with E
C is replaced with G
M N W are replaced with H

In the scheme used since 2002 replacing a number by a letter or a
letter by a number will not cause extra collisions.

The transformed plate number is then hashed with the one-at-a-time
hash function described here
http://www.burtleburtle.net/bob/hash/doobs.html

The 32 bit result is reduced to 24 or 18 bits simply by masking.

This is described in the UTMC Technical Guide TR007.001b which, as far
as I can tell is not published on the UTMC site.

If anybody would like a copy of the document please contact me off list.

It would appear that the Highways Agency's statement that a large
prime number is used is untrue.

I'm going to be doing some experiments to see how well the function
does with some generated numberplate data.

John Wilson