[Talk-GB] Postcode centroids

Andy Robinson (blackadder-lists) ajrlists at gmail.com
Fri Jan 21 10:33:27 GMT 2011

Richard Bullock [mailto:rb357 at cantab.net] wrote:
>Sent: 20 January 2011 11:10 PM
>To: talk-gb at openstreetmap.org
>Subject: Re: [Talk-GB] Postcode centroids
>> Please do not just add the centroid to the map. I don't see the value
>> of that. I am interested in the experience people gain from using this
>> data, for example to add postcodes to an address such as addr:postcode.
>I've added a few addr:postcode to my existing addr:* house numbering. In
>the vast majority of cases, I've found that the centroid itself isn't
actually a
>centroid in the geometric sense - but more sort of a "median" house.
>e.g. even on a curved road, the "centroid" lies at the position of one of
>houses that the postcode applies to, not at the geometric centroid - which
>would be some distance away and make it very tricky to assign post-codes to
>The following seem quite common in my area;
>*    Short roads often have just one post code covering every address on
>*    Slightly longer roads will often have one post code for one side of
>road, and one for the other.
>*    Some roads will be split into segments where each address on both
>of the road have one postcode - then further up the road, each will have a
>different one etc.
>*    Then there are ones with one for each side of the road - and split
>The roads mostly seem to split into segments if there is a convenient
>splitting up the housing - but sometimes not - and this requires a bit of
>educated guesswork sometimes.

I concur with all your observations. Where postcodes don't change at a
connecting street (or connecting track or footpath in some cases I've
observed) or to work out the postcodes for houses opposite a connecting
street mostly I've found by taking a central distance between the two
adjacent codepoint nodes it's possible to generally get the right postcode,
though I air on the side of caution and tag the building at the centre and
sometimes the house either side of this with a addr:postcode=? If I'm not
convinced. I'm finding that out of around 2000 properties tagged with a
postcode to date I've got perhaps a dozen I've left a ? on. Not bad really.

One thing that is absolutely clear to me is that you can't add postcodes
unless you have been around and assigned the right houses to the right
streets. On older  traditional streets where this is clear from imagery it's
not so difficult but in new housing estates it's impossible unless you have
walked the patch first. After that it all drops into place really rather



>What I've done in the past is created a .osm file with my particular
>of interest - and then added the file as a new layer in JOSM - so I can
>it. I would certainly be interested to use this new site - when my postcode
>areas of interest have been loaded.
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