Geometry

The line geometry provided for each USRN is an aggregation of multiple links to ensure that there is a 1:1 relationship between the USRN and a feature. Each USRN will be represented by a single line feature and each line feature will have a single USRN identifier.

OS Open USRN will include geometry sourced from both Ordnance Survey and Roads or Highways Authorities. Where possible, the geometry of streets captured by a Roads or Highway Authority is spatially matched to the geometry of OS Road Links and Path Links. Where this match is successful, OS geometry is used as the source geometry to represent the extent of the Street. Where spatial matching cannot match the geometry captured by the Roads or Highway Authority to OS geometry, the Roads or Highway Authority geometry is used as the source geometry to represent the extent of the Street. Using a combination of geometry sourced from OS and the Roads or Highway Authorities results in all USRNs being spatially represented in OS Open USRN.

Geometry simplification

Geometry simplification is the process of reducing the scale and complexity of the data whilst maintaining the important elements and characteristics of the features. The line geometry provided for each Street has been simplified to ensure that it provides the most suitable geometry for the purposes of openly sharing and linking information against the USRN, whilst ensuring the integrity of the USRN representation.

This simplification means that:

• The number of vertices representing the curvature of streets has been reduced.

• Short cul-de-sacs and slip roads are not supplied.

• Complex configurations of the road network are represented by a simplified depiction, such as large roundabouts, complex junctions and traffic islands.

This simplification process will never result in the loss of complete USRNs; all features with a unique USRN will not be removed.

Due to the generalisation of the geometry, we recommend a viewing scale range of 1:15 000 to 1:30 000.

Overlapping geometry

It is to be expected that there will be multiple representations of a piece of road or path. Where Type 3 and Type 4 USRN geometry exists, they will overlap Type 1 or Type 2 geometry. For example, Alan Drayton Way is represented by a Type 1 geometry (Alan Drayton Way) and a Type 3 geometry (B3037).

In England, overlapping USRN geometries may also occur where Highways England have an additional USRN allocated for the same street. This is due to Highways England being the responsible authority for trunk roads in England and they will therefore assign USRNs accordingly, whilst Local Highway Authorities will capture all the streets within their authority area, even if they are not responsible.

Additionally, multiple USRN geometries may exist for the same street where the Local Highway Authority boundaries are defined by the centreline of the street, where the geometry will be represented as overlapping.