Due to different Output Area (OA) polygons used for higher area assignment between NRS and ONS there will be differences in output produced in both the SPD and ONSPD, and SSPL and NSPL.
An example is shown in the table below, the postcode has the same grid reference in both products, however different OA allocations:
| Postcode |
Grid Reference Easting |
Grid Reference Northing |
SPD/SSPL
Output Area 2022 |
ONSPD/NSPL
Output Area 2022
|
| DD8 4PB |
337983 |
754753 |
S00139649 |
S00139648 |
One of the requirements for Output Areas were that they should be contiguous, unless caused by water. Unfortunately due to the nuances of postcodes and addresses this was not always possible.
It was agreed that for any non-contiguous OA that was not caused by water we would remove the parts and the area would be merged with an adjoining OA to create a single polygon where possible (in most cases this is the smaller of the 2 (or more) parts of the non-contiguous OA).
This did not affect the census statistics, calculation of the master postcode, the ‘higher area’ assignment, or the area calculation for the OA all of which were based on all of the constituent parts of the OA.
In the following example, the Output Area (S00139649) is non-contiguous in the image on the left. The master postcode for the OA is in the larger part of the polygon. In the image on the right we have split the output area spatially and the smaller polygon (the polygon that does not hold the OA grid reference) has been merged into the adjoining Output Area (S00139648).
If you refer back to the table, you can see why the OA allocation has different results between SPD/SSPL and ONSPD/NSPL based on the same postcode grid reference, due to the use of the ‘All parts’ vs ‘with parts removed’ boundaries.
Of the 46,363 2022 Output Areas created, 531 were non-contiguous because the polygons were separated by other Output Areas. A boundary dataset detailing the OA parts which were ‘removed’ is available on the NRS website should users want to create an ‘all constituent parts’ version of the 2022 OA boundaries.
Statistics on Differences
When NRS create the SPD and SSPL we want to use the most accurate depiction of the boundaries which is why we use the Output Area boundaries with all parts.
We have compared both directories (SPD 2025/1 and ONSPD Feb 2025) to highlight differences between the products. To make the figures more comparable, we have removed B/C splits, and postcode history from the SPD Index, and only chosen records with the country code (S92000003) from the ONSPD.
There are postcodes that exist on one directory but not the other, and vice versa. Over 99.9% of live postcodes match between the directories.
Of the live postcodes on the SPD that had a matching live postcode on the ONSPD we found that 99.72% matched on 2022 Output Area allocation. Where the OA allocation didn’t match (0.28%), 6.86% was due to the differing OA boundaries used by NRS and ONS, and 93.14% was due to different postcode grid references as a result of differing digitising methods, and data imputation.
Contact details
If you need any further information please contact our Geography Customer Services team.
