[Talk-GB] Postcode data

[Andy Allan]

On 26 February 2013 22:08, Aidan McGinley
<aidmcgin+openstreetmap at gmail.com> wrote:
> is the actual output that would get loaded onto OSM.

Please don't load this data into OpenStreetMap. It's not a good idea.

1) The source data appears to be heavily overprocessed.

"Users should note that postcodes that straddle two geographic areas
will be assigned to the area where the mean grid reference of all the
addresses within the postcode falls."

So while you're trying to map postcodes to a particular building in
OSM, what's actually happening is that the real postcode locations are
first being averaged to a centroid, then that postcode centroid is
assigned to a given geography (e.g. a LSOA, or whichever geography you
are using), and then you're taking the centroid of the geography (not
the centroid of the postcodes) and finding a random building in OSM
that overlaps that geography centroid, then adding the postcode to the
building. So you're adding postcodes to whatever building just happens
to be at the centroid of the geography, when all we know is that the
centroid of the postcodes is somewhere within that geography.

Having postcode data in OSM is useful, but this appears to be very
haphazard. There's no guarantee that the given building is anywhere
near the postcode centroid (the postcode centroid could be at the edge
of a given geography) and it's no surprise that each geography could
have multiple postcode centroids.

There are other approaches. We have access to postcode centroids from
elsewhere, if we were to pick just one building per postcode to assign
a postcode to, it would be better to use the centroid of the
postcodes, rather than the centroid of a geography that the centroid
of the postcodes happens to fall within.

2) The license is unclear

"ONS Intellectual Property in the postcode products is supplied under
Open Government Licence terms (see Related Links)."

Sure, OGL, great, but...

"The ONSPD is a Gridlink® branded product that pulls together data
from members of the Gridlink® Consortium (Royal Mail, Ordnance Survey,
National Records of Scotland, Land & Property Services (Northern
Ireland) and ONS)."

So the ONS might be happy to put their own IP (presumably the act of
mapping postcode centroids to geographies) under OGL, but as it says
above there's a bunch of other IP rights in the database, and the ONS
makes no statement on the licensing of the data.

3) We don't want to import this stuff anyway

Postcode centroids have been discussed many times before, and the
position we've taken is that importing them does not help our mappers.
It's derived data, not the kind of thing that we actually map. We use
the centroids in various visualisations and QA tools, we can expand
them out to voroni polygons to help figure out what the real postcodes
might be, but what we're aiming for is for buildings to be assigned
the *correct*, actual postcodes.

Until we get some real, full detail, all 28m buildings, data (e.g. the
PAF) under a suitable license, then please don't import centroids or
anything derived from them.

Cheers,
Andy