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 of 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 centre (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 9,550 or so postcode sectors we will have the basis for mapping and analysis.
The YO postcode area with an example district (YO23), and sector (YO23 2)
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 where you see how complex the boundary really is.
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.
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 9,551.
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 and 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, already this year we have edited well over 500 sectors and we continuously review the position of the sector, district and area labels to optimise the aesthetic quality of the mapping display at most scales.
Above: The light blue is one sector, but the separate "island" of light blue makes defining one boundary tricky.
Above: Postcodes in a sector at Scarborough. Note the one on the pier.
Our other blogs
Cycle to work day
Each year for #cycletoworkday we take a look at cycling statistics across the country and try to map that data and find interesting trends. This is mainly because we at Beacon Dodsworth are either a little bit obsessed about cycling, or we tend to worry about the environment.
Administrative geography is a way of dividing the country into smaller sub-divisions or areas that correspond with the area of responsibility of local authorities and government bodies. It provides an alternative to postcode geography but because it tends not to be used by consumers, it is often overlooked. We take a look at administrative geography, what it is and how to use it.
Mapping for local projects
Recently, we were contacted by a company responsible for organising charity door knockers. They needed more than 9,000 postcode sectors mapped at A4 size to pass to ground staff showing street level detail. This would enable them to use maps at a local level to plan fundraising routes and clearly define territories for each agent.
What is geodemographic profiling?
More than 64 million people live in the UK, each with their own outlook, priorities, needs and way of life. Geodemographic profiling offers a way to group these individuals to try and identify the right audience for your product or service.
As a Yorkshire-based company, we wanted to help celebrate Yorkshire Day, which takes place on 1st August. Naturally, we wanted to put a geographic spin on the celebration, so we took a look at drinking preferences within God’s own county.
All you need to know about postcodes but were afraid to ask
The humble postcode has been around for years. We look at how postcodes are used and what led to their introduction.
Google Fusion Tables
After almost 10 years of service, Google are to retire their Fusion Tables product at the end of 2019. This tool was very useful got visualising and sharing large amounts of tabular data - particularly amongst small and mid-sized businesses. So what can we do to fill the gap left by this tool?
The foundations of geographical analysis
Displaying data on maps makes it easier to understand as well as giving a new perspective on a problem. Using a GIS to prepare and present data has become increasingly popular over the last 20 years, but graphical displays of data on maps were around long before computers came along.
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Social change over 10 years
The next census isn’t due to take place until 2021 so we thought it was a good time to take stock of some of the changes and trends we noticed between the 2001 and the 2011 census. What difference does 10 years make to our society and the people that live within it?
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The power of postcode sectors
Postcode sectors are aggregations of individual postcodes and they provide meaningful geographical reporting areas in any GIS. However, they aren't as easy to map as you might think. Here is how we do it.
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Geodemographics and the University of East London
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