# Product Details

## Feature Content <a href="#bookmark2" id="bookmark2"></a>

The TOIDs included are drawn from the following feature tables from the following OS MasterMap products:

### OS MasterMap Topography Layer

* TopographicArea
* TopographicLine
* TopographicPoint

### OS MasterMap Sites Layer

* Site Extent

### OS MasterMap Highways Networks – Roads

* RoadNode
* RoadLink
* PathLink

All features from each feature type above have been included in the OS Open TOID.

For each of the feature types included the following method of generalisation is used for the location:

* Point Features: the original location round to the nearest meter.
* Lines Features: the midpoint of the line rounded to the nearest meter.
* Polygons Features: use a point of inaccessibility algorithm to find a point that is guaranteed to be with in the polygon which is farthest from all edges. This location is then rounded to the nearest meter.
* Multi Polygon Features (Site Extent): use the same method as for polygon features on the largest of the polygons.

## Coordinate Reference System (CRS) <a href="#bookmark3" id="bookmark3"></a>

The OS Open TOID and its associated geometry data layer uses the British National Grid (BNG) spatial reference system. BNG uses the OSGB36 (EPSG 27700) 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.

Introductory material on the BNG and Ordnance Datum Newlyn (ODN) along with the full definition of the BNG (OSGB36 National Grid) is available on the Ordnance Survey OS Net GPS site: <https://www.ordnancesurvey.co.uk/gps/transformation/>

[A Guide to Coordinate Systems in Great Britain](https://docs.os.uk/more-than-maps/deep-dive/a-guide-to-coordinate-systems-in-great-britain) contains more details on working with coordinate systems.

A general introductory guide to BNG is provided at: <https://getoutside.ordnancesurvey.co.uk/guides/beginners-guide-to-grid-references/>


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