> 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/transport-network-portfolio/os-open-roads/os-open-roads-overview/os-open-roads-data.md).
# OS Open Roads Data
## Simplification
The detail within OS Open Roads is automatically generalised from Ordnance Survey large-scale data. Simplification is the process of reducing the scale and complexity of data whilst maintaining the important elements and characteristics of the location.
OS Open Roads simplification comprises the following processes:
* **Selection/omission** – Some features that display at higher resolutions are removed at the lower resolutions. For example, cul-de-sacs of less than a specified length are not supplied.
* **Simplification of geometry** – Simplification can take several forms in OS Open Roads. Examples include reducing the number of vertices that represent a curve and representing a roundabout below a certain size with a RoadNode that is specifically attributed as a roundabout.
## Coordinate reference system
The Geography Markup Language (GML), shapefile, and GeoPackage product formats enable the use of the British National Grid (BNG) coordinate reference system. In the GML data, this is represented by reference to its entry in the EPSG registry, as: [http://www.opengis.net/def/crs/EPSG/0/27700.](http://www.opengis.net/def/crs/EPSG/0/27700)
The BNG spatial reference system uses the OSGB36 geodetic datum and a single Transverse Mercator projection for the whole of Great Britain. Positions on this projection are described using easting and northing coordinates in units of metres. The BNG is a horizontal spatial reference system only; it does not include a vertical (height) reference system.
The vector tiles format is in the Web Mercator (EPSG:3857) projection. This is a global coordinate reference system.
## Currency
OS Open Roads is derived from Ordnance Survey large-scale data and is refreshed every six months.
## Completeness
Quality control procedures are undertaken at all stages of production to ensure that the data is accurate, complete, and conforms to the specification. Quality control checks include automated data testing against the product specification and visual checks by operators.
## Precision
OS Open Roads features are published with geometry given to a precision of two decimal places.
---
# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.
## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:
```
GET https://docs.os.uk/os-downloads/products/transport-network-portfolio/os-open-roads/os-open-roads-overview/os-open-roads-data.md?ask=&goal=
```
`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.