# UK Counties mode: boundaries and population

This mode contains all **218 current administrative areas** in the Office for
National Statistics (ONS) **Counties and Unitary Authorities, December 2025**
geography. “UK Counties” is a short mode label, not a claim that every included
area is a historic or ceremonial county.

| Nation | Areas | Administrative geography |
|---|---:|---|
| England | 153 | 21 nonmetropolitan counties, 63 unitary authorities, 36 metropolitan districts, 32 London boroughs and the City of London |
| Wales | 22 | Principal areas (counties and county boroughs) |
| Scotland | 32 | Council areas |
| Northern Ireland | 11 | Local government districts |

The complete current geography is used: for example Kent excludes the separately
listed Medway unitary authority, and London boroughs are separately playable.
Historic counties are not mixed with current administrative boundaries.

## Primary boundary source

[ONS Counties and Unitary Authorities (December 2025), UK BGC](https://www.data.gov.uk/dataset/563f5079-796d-4190-ac7f-49219a6de896/counties-and-unitary-authorities-december-2025-boundaries-uk-bgc),
released May 2026. The BGC product is generalised to 20 metres and clipped to the
mean-high-water coastline. Features were downloaded from the official ONS ArcGIS
service as EPSG:4326 GeoJSON. One invalid source geometry, Nottinghamshire, was
repaired with Shapely `make_valid` before masking. Display outlines are further
simplified according to the map extent; population masking uses the full BGC
geometry before this display simplification.

Boundary licence: [Open Government Licence v3.0](https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/).
[ONS attribution requirements](https://www.ons.gov.uk/methodology/geography/licences):

> Source: Office for National Statistics licensed under the Open Government Licence v.3.0
>
> Contains OS data © Crown copyright and database right 2025

## Population and resolution

Population is the **European Commission JRC GHS-POP R2023A, epoch 2020** gridded
estimate, not a present-day census figure. The same global native 30-arcsecond
EPSG:4326 population raster is used throughout the game. A native cell is about
0.93 km north–south and approximately 0.46–0.60 km east–west in the UK. Native cell
centres inside each administrative boundary determine the included population.

Small areas keep the native grid: City of London has only five positive native
cells. Those cells are genuine source estimates; no finer population detail is
invented. A line crossing a cell is scored using its fraction of that rectangular
cell, so tiny-area results have limited spatial precision. Larger areas sum
native cells into coarser fixed cells to keep downloads small. Largest-remainder
rounding preserves each area's rounded source population total without negative
weights or invented city weighting.

The resulting population is **2020 population within December 2025 boundaries**.
The catalogue sums to 66,595,139 modelled residents across the UK. This number
must not be labelled an official 2025 or 2026 population count.

Population source: [GHS-POP R2023A](https://human-settlement.emergency.copernicus.eu/ghs_pop2023.php),
DOI [10.2905/2FF68A52-5B5B-4A22-8F40-C41DA8332CFE](https://doi.org/10.2905/2FF68A52-5B5B-4A22-8F40-C41DA8332CFE),
licensed [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).

Citation: Schiavina M., Freire S., Carioli A., MacManus K. (2023). GHS-POP R2023A.
European Commission, Joint Research Centre (JRC).

Place labels are genuine Natural Earth 1:10m populated places, public domain,
used only where the point lies inside the selected administrative area. Labels
do not generate or influence population weights. Natural Earth's smaller-place
coverage is incomplete, so many boroughs have no place label.

## Files and verification

`catalog.json` lists all 218 maps, including their ONS code, nation, scope and
population. The `file` field is relative to `/data/`, for example
`uk-counties/uk-kent.json`. Each map payload carries its source, licences,
resolution, 2025 boundary epoch and 2020 population epoch. `provenance.json`
records the exact input URLs and SHA-256 checksums.

Rebuild with `scripts/data-uk-build.py`. Run `scripts/data-uk-verify.py` for
structural, geographic, total, resolution and licence checks. Supply both source
caches to independently re-extract and compare all 218 native population totals.