Background
The geography of Scotland comprises the mainland plus many islands. Some of these islands are inhabited and through time their inhabited/uninhabited status can change.
Most of Scotland’s islands are found in three main areas, Orkney Islands, Shetland Islands, and the Hebrides. However, there are also clusters in the Firth of Clyde, Firth of Forth and within bodies of fresh water such as Loch Lomond.
For National Records of Scotland, the prime reason for identifying inhabited islands is to aid the delivery and collection of Census questionnaires.
If an island is inhabited, then Royal Mail (RM) will allocate postcodes to the island’s addresses.
To ensure that Census questionnaires are collected and delivered correctly, it is important that both parts of the postcode (the island addresses and the mainland addresses) are contained within the same Census Planning area (previously known as Enumeration District). The identification of inhabited islands helps with the creation of meaningful Census Planning areas.
In addition to the planning aspect of the census, there has always been an interest in the statistics associated with Scotland’s inhabited islands and the NRS Island dataset helps ensure that they are all identified.
How NRS define islands
NRS Geography define islands based on the Royal Mail Postcode Address File. If there is a live small user postcode on an island it is deemed inhabited.
Royal Mail are responsible for assigning addresses to postcodes to aid efficient mail delivery, they do not however, recognise administrative area boundaries. As a result, some postcodes may contain addresses in more than one Council area (including those split by the Scotland/England border), island/mainland of Scotland and/or island/island area.
NRS Geography draw postcode boundaries which follow physical features on the ground and which fit exactly within Council areas. As Royal Mail do not recognise administrative boundaries, NRS Geography split postcodes.
Split postcodes are identified on the Scottish Postcode Directory’s postcode index (Split Indicator field = Y), and the postcode label will have an ‘A’, ‘B’, or ‘C’ suffix.
Split postcodes occur when:
- A postcode straddles two or more Council area boundaries.
- A postcode straddles the Scottish/English border. These only get an ‘A’ suffix.
- An island and mainland share address content.
- Two islands share address content.
By splitting postcodes, it allows NRS to produce statistics that are exact-fit for Council areas, and at Census periods at individual island level for population and household counts.
More information on the NRS Split Postcode Policy is available in the Geography Policy section of the NRS website.
Many of the larger islands, with relatively large populations, will have postcodes that cover only the island. There are however occasions where an individual postcode will cover an island.
Whole Islands
There are 189 live small user postcodes (as at 2022/2 SPD) on the Isle of Arran, and 1 live small user postcode on Holy Island.
However, some of the smaller inhabited islands have very few households and in some cases the postcode for these island addresses is the same as that of some households on the mainland or another island.
Island/Mainland of Scotland
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). The pink and blue postcode boundaries show that NRS Geography have split the postcode to ensure that the island is showing as showing as inhabited.
How the base Island map was created
The island map is based on Ordnance Survey (OS) High Water Polyline (HWP) as at October 2018 BoundaryLineTM.
NRS Geography converted this to a polygon and cross-referenced this boundary against the existing Island map to ensure no islands were lost in the update. The dataset was then checked to ensure all RM Postcode Address File (PAF) addresses were within the new boundary, where there were issues, they were investigated, and the NRS Island boundary updated.
As NRS Geography require individual island level data, we removed connections (e.g., causeways) between individual islands that are on the OS HWP as shown in the next example.
More detailed information on the process for the Island map is provided within the ZIP file for each Island map release on the Islands section of the NRS website.
Change to the Island map
NRS Geography currently have no plans to recreate the Island map from a new OS HWP base.
We will, however, update our Island map for the following reasons:
- As the inhabited status for Islands is based on live small user postcodes, the status can fluctuate over time.
All islands that have never had a postcode are coded as 999. Should one of these get a postcode in future it will get the next sequential code, at the time of writing this it would be 181.
- OS Positional Accuracy Improvements.
New aerial imagery flown as part of Ordnance Survey’s National Cyclic
Revision allow them to correct the error on their datasets, which in turn allow us to provide a more accurate island map.
- Ad-hoc quality assurance to ensure no addresses are falling out with the Island boundaries.
Identifying whether an island is inhabited
From 2023/1 Scottish Postcode Directory (SPD), the Island map and corresponding lookup no longer provides the inhabited status.
Users can find inhabited islands using the postcode index by selecting all live digitised small user postcodes (Date of Deletion and Never Digitised fields are null) and the island code.
When a new island (that has never had an island code before) is introduced it will be highlighted in the relevant SPD Bulletin, and an updated version of the Island map will be made available.
