The power of postcode sectors

We all know our postcode, or at least we should do. It is an integral part of our address, making routing our mail (and increasingly, other deliveries) more efficient, and guiding SatNavs. Part of the power of the postcode comes because individual, full postcodes (e.g. YO23 2BD) are the building blocks of larger units: postcode sectors (e.g. YO23 2), districts (e.g. YO23) and areas (e.g. YO). They form a neat hierarchy - see our postcode blog for more on this.

Often our postcode is shared by a few neighbours (on average about 15). A postcode is a collection of addresses, usually on a single street, normally forming a natural group or cluster. The postcode sector is the most practical reporting/aggregated unit - small enough to give geographical detail but large enough to show some homogeneous behaviour and reveal national patterns.

Creating meaningful boundaries

Postview, Beacon Dodsworth’s postcode boundary product brings together raw postcode data into these meaningful analytical areas. It forms the backbone of some of our other products, for example Prospex for mapping, and TimeTravel for drive time lookups and Where’s My Nearest applications. It is also added in to some of our other clients' GIS systems to provide a foundation for robust area analysis.

Our product is based around The National Statistics Postcode Directory (NSPD), created by the Office for National Statistics (ONS). It supplies a single location for each postcode (equivalent of Ordnance Survey CodePoint Open). This is usually the location of the single address that is closest to the centroid of all the addresses that share that postcode. When we look at a group of postcodes with the same code except for the last two letters (i.e. all the postcodes in a single postcode sector) we see that they naturally form an area with a fairly clear boundary.

If we compute the boundaries for all the 11,200 or so postcode sectors we will have the basis for mapping and analysis.

The YO area with an example district and sector.
The YO postcode area with an example district (YO23), and sector (YO23 2)

Map of the YO23 2 postcode sector.

The postcodes that form the YO23 2 postcode sector.

A more complex postcode sector: SW18 1

Not all sectors are so easy to define, such as SW18 1, shown here.

The difficulties of defining boundaries

Of course it isn’t easy, there are several complicating factors:

1. Where postcodes meet very closely, the boundary has to "weave" between two dense sets of points. For example the maps below shows groups of postcodes that appear easy to put a boundary around, until you zoom in and see how complex the boundary really is.

An illustration of the difficulties of mapping a boundary around tightly-packed postcodes.

Weaving a boundary between densely-packed postcodes can prove difficult...


A zoomed-in illustration showing the difficulties of putting boundaries on postcode sectors

...especially when you zoom in and see how complex the postcode network can be.


2. Where postcodes are very spread-out, imposing a boundary can seem very arbitrary. For example, in the image below, should the grey dots form an "island" within the blue dots? Equally, where postcodes overlap, it is difficult to find a best fit that works for both accuracy and aesthetics.

An illustration of the difficulties of putting boundaries around spread-out postcodes.

Widely-spaced postcodes: should the grey points above form an island, or run flush with the neighbouring green point set.

3. Inaccurate locations - these are identified as "lone" postcodes inside another sector, for example, should the light blue rogue location to the right be an "island"?

4. Changes over time such as: improved accuracy of co-ordinates, new builds, demolition, reassignment - when a postcode is altered by Royal Mail, and reuse - when an "old" postcode is brought back into use at a different location.

Sometime there are not enough available postcodes for a sector. Because of the naming convention where the last two letters identify the postcode, there are only 400 available distinct postcodes (the letters C, I, K, M, O and V are not used because of easy confusion with other letters for automatic sorters and humans). In this case a new postcode sector is created. Sometimes the Royal Mail split a sector into two or more new ones.

From PostView’s point of view, new sectors emerge gradually as co-ordinate information is inserted into the NSPD. The first version of PostView, more than 20 years ago, had 8,400 postcode sectors. We now have 11,224 (as at November 2020).

5. "Large users", usually businesses, provide their own forms of complexity. A large complex, with its own postcode may exist inside the boundary of a residential postcode sector. Alternatively, their postcode may only be loosely related to their nominal sector, or they may be entirely stand-alone with no equivalent residential postcodes sharing their sector.

6. The coast forms a natural boundary for some sectors, but which coastline should we use? There are several suppliers of mapping information whose coastlines differ slightly. There is not always a single coastline from any one supplier but variations dependent upon tides - high, low and mean water levels. Sometimes details such as piers and docks appear in detail, sometimes not. We have “stitched” on the detailed Ordnance Survey Open Data Boundary Line coastline, which is Mean Low Water. This has the advantage of overlaying well with other OS background maps and it can be generalised (reproduced at a lower resolution, with fewer “wriggles”) for more rapid display (e.g. for online mapping).

7. Postcode sectors whose postcodes have not yet been assigned coordinates.

8. The Channel Islands: Jersey (JE) Guernsey (GY), and the Isle of Man (IM) have a postcode system but full postcode co-ordinates are not published. We have created sectors from locality information and royalty-free maps. For mapping convenience we have transposed the Channel Islands north in the English Channel.

Because of the changes over time we run a regular health check to identify sectors whose constituent postcodes lie outside their boundary. We amend the boundaries where the changes have been greatest, to ensure that Postview is as accurate as we can make it. For example, we recently edited well over 500 sectors and continuously review the position of the sector, district and area labels to optimise the aesthetic quality of the mapping display at most scales.


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A split postcode sector with a separate "island" of postcodes.

Above: The light blue is one sector, but the separate "island" of light blue makes defining one boundary tricky.

Postcodes around Scarborough - including on the pier.

Above: Postcodes in a sector at Scarborough. Note the one on the pier.

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