[Talk-GB] Postcodes

Fri Nov 9 14:44:46 UTC 2018

I'm pretty sure that the "centroid" is allocated to the nearest delivery
point in the postcode which places it over a building. See my (now rather)
old blog post
<http://sk53-osm.blogspot.com/2013/12/british-postcodes-on-openstreetmap.html>
from 2013, and the note by Jenni Tennison. A caveat is, of course, that the
Land Registry Prices Paid data proved to be an open data mirage.

Please remember that Nominatim has a table (not recently updated) of all
postcode centroid which are used for searches. These usually show as AB10
2## or similar and are at a lowish zoom level.

Judging by taginfo stats we now have around 8-10% of all postcodes mapped,
and Robert Whittaker's site suggests
<https://osm.mathmos.net/addresses/pc-stats/> over 10%, so better than in
2013, but nowhere near the level we could get if we adopted a sustained
campaign to use what information we have.

Personally, I add addr:postcode to streets when: a) it is clear that all
properties share a postcode, but individual properties have not been
mapped; and b) when the local authority includes the full postcode on the
streetname sign (e.g., Gedling & Rushcliffe). In the former case this
should be regarded as an iterative step towards the desired position of
individually mapped addresses; in the latter it reflects an on-the-ground
rule.

The available sets of open data which can be used to resolve postcodes are:
Food Hygiene (the best, easiest to resolve, coverage of the whole UK - even
Rutland); Companies House Open Data (surprisingly useful even in areas of
social housing); the National Register of Social Housing (NROSH, not
updated since 2011, but still very useful); CQC (medical practices, care
homes etc). I haven't looked to see how many postcodes are covered by these
in total, but it should be a reasonable proportion of the total. If you
aren't aware Will Phillips OSM-Nottingham site does allow searching of
various open data sets across the UK (I would recommend searching only in
the viewport, so you need to zoom out and in to the target area). The
quickest way to ensure at least one address is mapped for a given postcode
is using Greg's FHRS tools.

Jerry

On Fri, 9 Nov 2018 at 13:44, Adam Snape <adam.c.snape at gmail.com> wrote:

> Hi,
>
> I ask because the 'centroids' do not appear to be centroids in a pure
> mathematical sense, they always appear to be placed on a building, never in
> open space. Now, if this were merely been done by attributing the centroid
> to the nearest building regardless of whether it actually belongs to the
> postcode or not, it would serve no purpose. It seems far more likely that
> it would be attributed to the nearest building belonging to that postcode.
> If this is the case then it gives us a way of tying these centroids to an
> actual building within each postcode area and that gives us something
> tangible to map. Can anybody suggest whether I'm onto something here?
>
> Kind regards,
>
> Adam
>
> On Fri, 9 Nov 2018 at 13:27, David Woolley <forums at david-woolley.me.uk>
> wrote:
>
>> If centroid has the plain (mathematical) meaning of the word, it will
>> only fall exactly on the building centre if there is only one building
>> in the postcode area.
>>
>> In practice the building nearest the centroid might have its own
>> postcode, so you can't rely on the nearest building to the centroid
>> having that postcode.
>>
>> There are, at least theoretically (e.g. a C shaped postcode) where the
>> centroid is in an adjoining postcode.  I imagine you would get this if
>> there was a cul-de-sac projecting into a crescent that was small enough
>> to have one post code.
>>
>> On 09/11/18 13:12, Adam Snape wrote:
>> > Hi,
>> >
>> > I agree with not mapping the centroids but...
>> >
>> > Is it the case that the centroids are always placed on a building which
>> > falls under that postcode? If so, wouldn't it be okay to tag the
>> > building with the appropriate postcode?
>> >
>> > Another idea: Given that postcodes (with few exceptrions) apply to only
>> > one street, would it be acceptable to add the postcode tag to the
>> street
>> > where there is only one centroid on the street?
>> >
>> > Kind regards,
>> >
>> > Adam
>> >
>> > On Fri, 9 Nov 2018 at 12:26, Tom Hughes <tom at compton.nu
>> > <mailto:tom at compton.nu>> wrote:
>> >
>> >     On 09/11/2018 11:44, David Woolley wrote:
>> >      > On 09/11/18 11:34, David Woolley wrote:
>> >      >> if you are only dealing with centroids, I think many have been
>> >     mapped
>> >      >> already,
>> >      >
>> >      > <https://taginfo.openstreetmap.org/search?q=uk_postcode_centroid
>> >
>> >      > indicates that at least 2500 have been mapped.
>> >
>> >     Yes, but it's a stupid idea, so please don't...
>> >
>> >     Tom
>> >
>> >     --
>> >     Tom Hughes (tom at compton.nu <mailto:tom at compton.nu>)
>> >     http://compton.nu/
>> >
>> >     _______________________________________________
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>> >
>> >
>> >
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