Currency
In 2017, the Census Design Authority approved National Records of Scotland (NRS) Geography’s recommendation to continue with the NRS frozen postcode geography policy which has been in place for censuses in Scotland since 1991.
Background
Royal Mail defines postcode areas for sorting mail efficiently; the postcodes have no relationship with administrative or electoral boundaries. Royal Mail requires a stable geography in order to deliver its services, which these areas cannot provide. As a result, some postcodes may contain addresses in more than one Council area.
NRS Geography have split postcodes for statistical reasons since we first began plotting postcode boundaries in 1973.
NRS split postcode policy has enabled census and other NRS statistical areas to produce better quality statistics. This is particularly true for census statistics which are produced exactly for Scotland, Council area, Health Board area, and Islands.
Split postcodes also ensure that the boundaries of the Scottish Government’s flagship geographies Data Zones and Intermediate Zones are exact-fits to Council areas since they are both derived from census output areas (amalgams of neighbouring postcodes).
What are split postcodes?
NRS Geography draw postcode boundaries which follow physical features on the ground, and which fit exactly within Council areas. This is not always possible as Royal Mail do not follow Council area boundaries when assigning addresses to postcodes or creating new postcodes.
In these situations, Geography draw the postcode as a separate polygon on either side of the boundary and add a suffix to the end of the postcode to distinguish between the different parts. The most populous part of the postcode is identified by suffix A and the smaller parts by suffixes B, C, etc.
The Royal Mail Delivery Point Count (DPC) allocated to the whole postcode is re-distributed between the parts of the postcode, the A part being assigned to the area with the largest address content.
The split indicator field in the Scottish Postcode Directory (SPD) is populated ‘Y’ which makes split postcodes easy to identify.
Split postcodes are those with an ‘A’, ‘B,’ or ‘C’ suffix and occur when:
A postcode straddles two or more Council area boundaries
As shown in the example below there is address content for postcode G33 1SH which falls across the East Dunbartonshire Council area and Glasgow City Council area boundary (the thick black line). NRS Geography have created two separate parts (the pink and blue boundaries) which are labelled G33 1SHA and G33 1SHB.
A postcode straddles the Scottish/English border
As shown in the example below there is address content for postcode TD9 0TU which falls across the Scotland / England border (the thick black line). NRS Geography created a boundary (shown in blue) which has been labelled TD9 0TUA. This identifies it as a split postcode, however as it is at the border, we do not create the corresponding boundary for what would be the ‘B’ part.
An island and the mainland share a postcode
As shown in the example below there is address content (shown as black dots) for postcode IV40 8DX which falls across an island (shown in orange) and the mainland of Scotland (shown in green) which while in the same Council area, are intersecting the postcode. NRS Geography have created two separate parts (the pink and blue boundaries) which are labelled IV40 8DXA and IV40 8DXB.
A postcode contains property on more than one island.
As shown in the example below there is address content for postcode ZE2 9LE (shown as black dots) which falls across two islands, East Burra (shown in green) and West Burra (shown in yellow) which while in the same Council area, are intersecting the postcode. NRS Geography have created two separate parts (the pink and blue boundaries) which are labelled ZE2 9LEA and ZE2 9LEB.
What happens with large user postcodes?
Large user postcodes do not have a digital boundary, this means that they will not have a split suffix. They can, however, be linked to a split small user postcode based on the grid reference of the large user postcode.
In the example below, the large user postcode for Crinan Hotel PA31 8SR (shown as a black dot) has been linked to postcode PA31 8SWA, which is a split postcode.
Maintenance of split postcodes
Whilst the number of split postcodes fluctuates from one quarter to another, overall numbers remain constant. Change occurs as a result of action by Royal Mail, who may add new addresses or remove addresses from the postcode, the result of which is to create a new split postcode or make an existing split postcode a whole postcode.
As at the 2022/2 SPD, there were circa 154k live small user of which 458 (0.3%) were split postcodes and of these six were split across the Scotland/England border. Of the circa 5,500 live large user postcodes, thirty-seven had their grid reference falling in a split small user postcode (0.6%).
NRS Geography IT created a split postcode system which is an integral part of several systems used in Geography to produce both SPD and SAD. The Geography systems are now mostly automated, which means that the resource required to maintain split postcodes is minimal.
Use Cases
Geography require split postcodes to satisfy current customer requirements. The following is a selection of internal and external stakeholder’s requirements for split postcode data availability at both postcode and address level
NRS Population and Migration Statistics
- Accurately estimate populations of Settlements and Localities. The need for accurate Settlement and Locality data feeds into the Scottish Government Grant Aided Expenditure calculations and the Urban/Rural Classifications.
- Project populations of Strategic Development Planning areas and National Parks. More accurate population projections were developed after feedback from users who requested this.
NRS Household Estimates and Projections
- Accurately produce household estimates at Data Zone level.
- Produce household projections for Strategic Development Planning areas and National Parks.
NRS Vital Events
- Accurately assign vital events births, deaths, marriages to the correct Council area for statistical reporting and other ad hoc purposes.
NRS Census
- Create planning areas which fit exactly within Council areas for Census operations.
- Produce exact census statistics for Scotland, Council area, and islands. Since Health Board areas now align to Council areas exact census statistics can also be produced for Health Board areas.
- Report on the status of islands (i.e., whether inhabited or not). Having this information allows Census to produce basic household and population counts at Census time for all inhabited islands in Scotland.
- Create Output Area boundaries (which are used to create higher geographies such as Data Zones and Intermediate Geography) which fit exactly within Council Areas.
External
- The Scottish Government for Survey and collection activities, creating Data Zones and Digital Scotland programme.
- HM Revenue & Customs to validate Scottish taxpayers. Enquiries have also come from the Scottish Government team taking forward new social security reforms in Scotland and their requirement is similar to HMRC.
- The Information Systems Directorate (ISD) integrate the SPD into ISD systems.
- The Local Government Boundary Commission for administrative and electoral review work.
- Processing ad hoc requests for geographic information at Council area level.
Impact of not splitting postcodes
On NRS statistics
The impact of not splitting postcodes simply means less accurate statistics – in other words a worse best-fit – and a fundamental change of NRS policy.
On Geography customers
We would not be able to meet current customer requirements, as detailed in the Use Cases.
On Geography systems
The systems created for the interlinked ‘Postcode Management’ and the ‘Address Register’ systems were developed with split postcodes as a fundamental requirement (2013 Review). To dismantle split postcodes from these systems would require significant redevelopment and testing of every component level.
It would cause more work for Geography to get rid of split postcodes and there is a danger that, if we removed split postcodes now but decide later that we need split postcode information, new systems and practices would have to be developed to obtain the same level of split postcode information we have now.
Do ONS split postcodes?
ONS do not split postcodes.
The ONS Postcode Directory (ONSPD) holds the postcode as a whole record.
ONS confirmed that their OA boundaries are drawn to fit exactly within Local Authorities (LA) but, rather than splitting the data for postcodes which straddle LA boundaries (which is what we do), they allocate all data to the part of the postcode containing the centroid (i.e., best-fit). This results in neat-looking boundaries which fit exactly within LAs, but statistics which are best-fits wherever the postcodes cross LA boundaries.
Policy Reviews
The requirement for split postcodes has been reviewed in the past; in
- 2008 for 2011 census purposes.
- 2013 as part of the Postcode Management and Address Register systems project.
- 2017 for 2022 census purposes.
Following each review, it was agreed to continue with our split postcode policy.
Further information
More information about NRS postcode boundaries and the Scottish Postcode Directory (SPD) is available on the Geography Policy page of the NRS website.
