> For the complete documentation index, see [llms.txt](https://docs.os.uk/os-downloads/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.os.uk/os-downloads/products/buildings-and-infrastructure-portfolio/os-open-linked-identifiers/os-open-linked-identifiers-technical-specification/street_usrn_topographicarea_toid_4.md).
# Street\_USRN\_TopographicArea\_TOID\_4
This page aims to provide all the information to evaluate if the method of associating the respective identifiers is suitable for your needs. These identifier relationships have been extracted from existing premium products and are presented in a standalone table for easy and open access to the information. It describes which product the information was extracted from, what information was extracted and how the source product derived the original association between the identifiers.
Identifiers displayed on OS Map data showing RoadLink and Topographic area feature types
## Relationship Extraction
This relationship is between the Unique Street Reference Number (USRN) of the Street features in *OS MasterMap Highways Network* and the TOID of the TopographicArea of the corresponding road surface area in *OS MasterMap Topography Layer*.
A USRN is a unique, persistent identifier for every street, assigned in the National Street Gazetteer (NSG) to any road, footway, path, cycle track, track or passageway that forms a highway. Where possible, the geometry of streets provided in the National Street Gazetteer is spatially matched to the geometry of OS RoadLinks to provide the additional attribution captured in the NSG against detailed OS geometry.
This relationship is extracted from the relatedRoadArea attribute on the matched RoadLink(s) within *OS MasterMap Highways Network – Roads* product following its release. This means that this relationship will not include all USRNs in *OS MasterMap Highways Network*, as Street features that have not been matched to an OS RoadLink will be omitted.
## Correlation Method
Where Street features have been successfully matched to OS RoadLinks, the Street features in *OS MasterMap Highways Network* are aggregate features composed of one or more RoadLinks. The relationship is determined by a line in a polygon intersection between the centre line geometry of matched RoadLink(s) and the topographic area polygon. The relationship is performed by Ordnance Survey.
The two source products used to extract this relationship are on different refresh cycles. *OS MasterMap Highways Network* is updated monthly, whereas *OS MasterMap Topography Layer* is updated every six weeks. Therefore, some of the version information in the product may have changed, meaning that the relationship is out of sync. A Confidence Value is assigned to indicate the reliability of the version information provided in this product in relation to the version of the features used to form the correlation in the original product. The Confidence Value is given for each relationship, which is defined in this relationship as:
| Confidence Value | Definition |
| ------------------------------------------- | ------------------------------------------------------------------------------------------------------------------- |
| Version information is correct | The version information provided is the same as the version of the feature used to create the correlation. |
| Version information has potentially changed | The version information could be different from the version of the feature that was used to create the correlation. |
| Version information has changed | The version information provided is different from the version of the feature used to create the correlation. |
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