Buckinghamshire CC ANPR cameras
tugwilson at gmail.com
Sun Feb 5 14:00:46 GMT 2012
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
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.
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