Background
NRS Geography digitise postcode boundaries to Total Extent. This means our postcode boundaries are full coverage, and there are no holes in our dataset.
The Total Extent boundary we digitise to is based on Ordnance Survey Boundary Line Council areas. NRS Geography only update the Council area boundary (and align postcodes to it) when there is an SI, changes created as part of OS continual improvement are picked up on when there is an official SI that requires us to update our Council boundary data.
When we create output for the Scottish Postcode Directory (SPD), we clip the boundaries to the coastline (Mean High Water Mark) and remove inland water (as listed in the Standard Area Measurements (SAM) produced by the Office for National Statistics).
More information on postcodes and production of the SPD is available in the Geography Policy section of the NRS website.
Postcode Content
The postcode boundary should encompass every address listed in the Royal Mail Postcode Address File as belonging to the postcode. In the example below, the address content for AB25 3TR ‘Powis Place, Aberdeen’ is
- 1 – Flats A to F
- 3 – Flats A to C
- 13 – Flats A to F, and
- 15 – Flats A to F
Minimum sensible boundary
The minimum sensible boundary is the area necessary to ensure all address content is contained while following ‘natural features’.
To assist in creating the minimum sensible boundary NRS Geography digitise postcodes as being either urban or rural.
For urban
The housing part of the postcode is contained within the minimum sensible boundary.
For rural
Scattered housing and farms will normally have fields and open land contained in the postcode polygon.
It is not always possible to achieve a clear distinction between urban and rural, but the minimum sensible boundary rule will always apply.
Split Postcodes
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.
NRS split postcodes for statistical reasons when creating postcode boundaries; they are not a feature of Royal Mail. Split postcodes are those with an ‘A’, ‘B’ or ‘C’ suffix and occur when:
- a postcode straddles two or more Council area boundaries. The most populous part of the postcode is identified by suffix A and the smaller parts by suffixes B, C, etc. (example 1);
- a postcode straddles the Scottish/English border, and the Scottish postcode is allocated a suffix ‘A’ (example 2);
- an island and the mainland share a postcode (example 3);
- or a postcode contains property on more than one island (example 4).
Our split postcode policy can be found in the Geography Policy section of the NRS website.
Multi Storey Postcodes
It is possible for large buildings with many separate delivery points (for example, a tower block) to have more than one unit postcode within the building. NRS Geography refer to these as multi-storey postcodes.
To distinguish between multi-storey postcodes and other postcodes they are drawn as concentric (non-overlapping) polygons.
The bottom postcode is drawn first, the second postcode is drawn within the boundary of the first, and so on, encompassing part of the building to ensure that postcode grid references are on the building.
Traditional high-rise buildings (blocks of flats)
The example below shows ‘Dalziel Tower’. There are two postcodes that encompass the addresses for the building, ML1 2HE – Flat 1 to 57 (shown in blue shading), and ML1 2HF – Flat 58 to 105 (shown in red shading).
Student accommodation
More student accommodation is being built in this style of property with a postcode per floor. An example of this is ‘Vita Student Glasgow, 21 Beith Street’ as shown below. There are 10 postcodes that encompass the addresses for the building:
- G11 6QL – Room 101a to 158,
- G11 6QR – Room 201a to 259,
- G11 6QS – Room 301a to 359,
- G11 6QT – Room 401a to 459,
- G11 6QU – Room 501a to 559,
- G11 6QX – Room 601a to 659,
- G11 6QY – Room 701a to 736,
- G11 6QZ – Room 801a to 823,
- G11 6RF – Room 901a to 917, and
- G11 6RG – Room 1001a to 1013.
Postcode boundaries that that are contained within another postcode boundary but are not multi-storey postcodes are attached to a neighbouring postcode as shown below.
Shared access buildings
There are many buildings/properties that share postcodes, street corners for example. In these cases, NRS Geography digitise a postcode by cutting through the property to avoid creating strange shapes.
In the example below there are address points that straddle two different postcodes. The address content for EH11 1SF is 1 to 10 Harrison Place, Edinburgh, and for EH11 1SG is 24 to 42 Harrison Gardens, Edinburgh. NRS Geography have digitised the boundary by cutting through the street corner to show that there is address content for each postcode on that particular building.
Non-contiguous postcodes
When postcode addresses are defined as areas, in some cases the layout of the addresses makes it impossible to create single postcode area for all the postcodes. This is because the layout of the addresses with different postcodes makes no difference to the delivery of mail by Royal Mail.
NRS Geography attempt to keep all postcode content within a single polygon, however this is not always possible, and we need to create a non-contiguous postcode.
A non-contiguous postcode is when the postcode cannot be digitised as a single polygon because it is separated by another postcode, water, or both postcode and water.
In the example below, EH24 9EN address content is ‘1 to 57 (odds) Lindsay Circus, Rosewell’ this is shown as a contiguous postcode boundary, shaded in purple. The address content for EH24 9EP is ‘2 to 58 (evens) Lindsay Circus, Rosewell’, this is shown as a non-contiguous postcode boundary, shaded in green.
The following example shows address content for postcode KA3 1PY that is divided due to another postcode (KA3 1PZ), this results in KA1 3PY being drawn as a non contiguous postcode (shown in blue shading).
The next scenario shows where the postcode boundary is divided by both surrounding postcodes, and water.