Product details

Coordinate reference system

The vector product formats allow for the use of a variety of coordinate reference systems. At present, only British National Grid (BNG) is used in OS OML vector and raster data. 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.

Currency

OS OML is derived from the latest versions of our OS MasterMap large-scale data and is refreshed every six months. More information about the OS MasterMap revision policy is available on the OS website.

Completeness

During production, numerous checks are undertaken to ensure that the data supplied to customers is both accurate and complete. During digital manipulation and data creation, all data sources are checked for conformance to specification.

These quality checks take the form of:

• Visual checks by operators.

• Data testing against the product specification.

• Data testing on a selection of tiles from the full national set.

Generalisation

Geometry within OS OML has been generalised from Ordnance Survey large‑scale data. Map generalisation is the process of reducing the scale and complexity of map detail while maintaining the important elements and characteristics of the geometry.

Map generalisation comprises the following processes:

• Selection/omission: Some features that appear at larger scales are not selected at the smaller scales. For example, in the public amenities layer, individual features in close proximity can be grouped to a single point.

• Simplification: Simplification can take a number of forms in OS OML, including line simplification. For example, in the vector product, a stream can have many curves; these curves are represented by a number of data points which are filtered and will maintain the feature's shape.

• Exaggeration: This is the process whereby features that are small but are considered too important to a particular landscape to be omitted are enlarged. For example, isolated rural buildings are often enlarged to a minimum size rather than being omitted.

• Aggregation: Aggregation is the combining of a number of small features to make a larger one, such as buildings.

• Symbolisation: Features that are shown in detail in OS VectorMap Local may be collapsed to points within OS OML, which can then be symbolised if required. For example, railway stations and airports are depicted as point symbols.

• Displacement: This is the movement of the representation of a feature away from its ground position in order to maintain its prominence. There is very little displacement in OS OML, but in certain circumstances, some features may be moved away from adjacent detail if their representation would otherwise be lost.

Edgematching

In the vector product, features that cross tile edges are included in both tiles, represented as 'hairy' tiles. This facilitates the creation of a seamless dataset and avoids the creation of invalid geometries from arbitrarily cutting features along the tile border. All features have unique identifiers which can be used to remove the duplicated features across tile edges when stitching the tiles together. This supports the greater use of the data in large-scale analytical applications.

These identifiers will not be persisted or maintained between product releases.