[Talk-GB] code-point_open accuracy

[Mark Goodge]

On 17/01/2023 09:38, Robert Whittaker (OSM lists) wrote:

> I assume that you're referring to "Large User Postcodes" here, which
> are postcodes allocated to specific businesses at specific premises
> that receive large quantities of mail. As well as banks,
> solicitors/lawyers often have them. At least with current practice,
> such postcodes should cease to exist if the business they were
> allocated to stops using the premises. However, there is sometimes a
> delay in the postcode being withdrawn, and the next occupant might
> mistakenly start using it as the postcode still existed for the
> premises at the time they moved in.

Yes; Large User postcodes are assigned to organisations rather than 
premises and don't transfer to subsequent occupiers of the premises.

Also, any organisation which has a Large User postcode will also still 
have a normal geographic postcode. It won't form part of their official 
Royal Mail assigned postal address, but it will still work if used. One 
common use case is for organisations with multiple premises - their 
postal address will be the main building, but they may have a separate 
address using the geographic postcode for courier deliveries that need 
to go to a different delivery point. (Ditto for sat-nav use, the Large 
User postcode of an organisation may not necessarily take you to the 
door you need to use to enter it, but the geographic postcode will 
usually get you close enough).

> If a business is claiming a postcode that isn't
> listed at all in Code-Point Open, then it's either a newly issued code
> that has yet to make it to Code-Point Open, or the business is
> mistaken and does not know its own postcode. Since postcodes are
> assigned by Royal Mail rather than being a choice of the business, we
> shouldn't give what the business says precedence over definitive
> information that originates from Royal Mail.

Typos are not unknown either, particularly on websites :-)

> As far as I know, Code-Point Open doesn't say which Postcodes are
> "Large User Postcodes". However, the ONS Postcode Directory does, and
> that is also an OSM-Compatible Open Data source that we can use.

The ONSPD also includes lookups to various additional administrative and 
census areas that aren't on Code-Point Open.

The ONS UPRN database includes postcodes, so if you know the UPRN of a 
premises then you can also look up the postcode. This is more reliable 
than using either Code-Point Open or the ONSPD, because it's based on 
actual postcode assignations rather than centroids.

One thing you have to watch out for, though, with the ONSUD is that it 
includes every UPRN issued so far, including those which have 
subsequently been withdrawn, and has no start/finish date for UPRNS. 
(Unlike the ONSPD, which also includes terminated postcodes but gives 
the termination date for them so you know which ones are obsolete). So, 
while it's reliable as a forward lookup from a known, active UPRN to a 
postcode, it's less reliable as a reverse lookup from a known postcode 
to UPRNs.

Mark