Geography codes and vintages

A GSS code identifies an area as constituted at a point in time. It is not a permanent identifier for a place, and treating it as one is the most common way a UK data join goes quietly wrong.

What we hold

The ONS Code History Database, loaded in full: every code with the date it became operative, the date it terminated if it has, and the successor relationships between them. We hold 22,836 code episodes and 33,542 successor relationships.

Rows are not codes

A code that is renamed appears in the register more than once. So the count of register rows and the count of distinct codes differ, and we publish both rather than picking whichever is convenient. For local authority districts: 471 register entries covering 461 distinct codes, of which 361 are currently live.

Where the Statutory Instrument is missing

ONS records an instrument against many changes, but not all. Entries carrying 1111/1001, 2011/1001 or 2021/1001 are markers for its own re-coding exercises rather than Orders. Only 47 of the 361 live local authority codes carry a real instrument. We show the rest as absent; presenting a placeholder as a citation would be inventing provenance.

Northern Ireland

This vintage of the Code History Database carries no live Westminster constituency codes for Northern Ireland. Our constituency counts therefore cover 632 of the 650 seats, and we say so wherever the number appears rather than publishing 632 as though it were the total.

Direction is asserted, not assumed

In the source, the column that looks like the predecessor is the successor. We verified the direction against known reorganisations and the build fails if a successor does not become operative on the date of its own change — because an inverted graph would state, confidently, that new areas were replaced by old ones.