# Technical specification

This technical specification explains the content of the OS Linked Identifiers API and the methods you can use to call it.

The OS Linked Identifiers API is a free OpenData product that allows you to access the valuable relationships between properties, streets and OS MasterMap identifiers.

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If you are not familiar with the OS Linked Identifiers API please read the [overview documentation](https://docs.os.uk/os-apis/accessing-os-apis/os-linked-identifiers-api) before proceeding.
{% endhint %}

## Core concepts

This specification does not cover core concepts that apply to all Ordnance Survey APIs.

Please read the following pages to learn more about OS API core concepts:

* [authentication](https://docs.os.uk/os-apis/core-concepts/authentication "mention")
* [error-codes](https://docs.os.uk/os-apis/core-concepts/error-codes "mention")
* [rate-limiting-policy](https://docs.os.uk/os-apis/core-concepts/rate-limiting-policy "mention")

## API reference

OS Linked Identifiers API has four main endpoints:

<table data-view="cards"><thead><tr><th></th><th></th><th data-hidden data-card-target data-type="content-ref"></th></tr></thead><tbody><tr><td><strong>Identifier</strong></td><td>Query by identifier and find its associated IDs.</td><td><a href="technical-specification/identifier">identifier</a></td></tr><tr><td><strong>Identifier Types</strong></td><td>Find associated identifiers from an identifier type.</td><td><a href="technical-specification/identifier-types">identifier-types</a></td></tr><tr><td><strong>Feature Types</strong></td><td>Find associated identifiers from a feature type.</td><td><a href="technical-specification/feature-types">feature-types</a></td></tr><tr><td><strong>Product Version Information</strong></td><td>Find version information for identifier relationships.</td><td><a href="technical-specification/product-version-information">product-version-information</a></td></tr></tbody></table>

### Correlation method table

The table below lists all of the correlation methods:

<table><thead><tr><th width="414">Correlation method</th><th>Feature relationship</th></tr></thead><tbody><tr><td><code>RoadLink_TOID_TopographicArea_TOID_2</code></td><td>RoadLink ↔ TopographicArea</td></tr><tr><td><code>Road_TOID_TopographicArea_TOID_3</code></td><td>Road ↔ TopographicArea</td></tr><tr><td><code>Street_USRN_TopographicArea_TOID_4</code></td><td>Street ↔ TopographicArea</td></tr><tr><td><code>BLPU_UPRN_TopographicArea_TOID_5</code></td><td>BLPU ↔ TopographicArea</td></tr><tr><td><code>RoadLink_TOID_Road_TOID_7</code></td><td>RoadLink ↔ Road</td></tr><tr><td><code>RoadLink_TOID_Street_USRN_8</code></td><td>RoadLink ↔ Street</td></tr><tr><td><code>BLPU_UPRN_RoadLink_TOID_9</code></td><td>BLPU ↔ RoadLink</td></tr><tr><td><code>Road_TOID_Street_USRN_10</code></td><td>Road ↔ Street</td></tr><tr><td><code>BLPU_UPRN_Street_USRN_11</code></td><td>BLPU ↔ Street</td></tr><tr><td><code>ORRoadLink_GUID_RoadLink_TOID_12</code></td><td>Open Roads RoadLink ↔ OSMM Highways RoadLink</td></tr><tr><td><code>ORRoadNode_GUID_RoadLink_TOID_13</code></td><td>Open Roads RoadNode ↔ OSMM Highways RoadLink</td></tr></tbody></table>

Please see the following documents for more information about how each of these identifiers has been linked to each other:

* [OS Open Linked Identifiers Technical Specification](https://docs.os.uk/os-downloads/identifiers/os-open-linked-identifiers/os-open-linked-identifiers-technical-specification)
* [Open Identifiers product family](https://docs.os.uk/os-downloads/identifiers/os-open-identifiers-family-information)
* [Open Identifiers support page](https://docs.os.uk/os-downloads/identifiers/os-open-identifiers-family-information)