OS MasterMap Topography Layer support

Please see the real-world object catalogue, which lists every type of real-world object in the OS MasterMap Topography Layer specification, and details how it is represented by Topography Layer features.

For cartographic representation, OS MasterMap Topography Layer is captured and designed for display at 1:1250, 1:2500 and 1:10 000 scales in urban, rural and mountain/moorland areas respectively. In GIS OS MasterMap can be viewed over a considerable range of scales. The most scale-sensitive feature types, such as cartographic text and symbol features, are fixed in size and rapidly become less clear at smaller display scales (that is, when zoomed out).

OS MasterMap Topography Layer is also suitable for use in non–cartographic applications where the concept of scale is less applicable.

We define accuracy in three ways:

• Absolute accuracy – how closely the coordinates of a point in the dataset agree with the coordinates of the same point on the ground (in the British National Grid reference system).
• Relative accuracy – positional consistency of a data point or feature in relation to other local data points or features within the same or another reference dataset.
• Geometric fidelity – the ‘trueness’ of features to the shapes and alignments of the objects they represent when testing the data according to the dataset specification against the ‘real world’ or reference dataset.

The following table represents the absolute and relative accuracy applicable to the scale at which the product was surveyed.

*RMSE (root mean squared error) is the square root of the mean of the squares of the errors between the observations.

In OS MasterMap Topography Layer, features are grouped into themes, such as buildings, land and water and so on, to enable more flexible data selection by customers. The rules that dictate which theme a feature becomes part of are governed by the classification attributes of the feature. These rules are in OS MasterMap Topography Layer themes chapter of the user guide.

Full supply and change-only update (COU) data will contain all of the themes in your contract area.

For topographic area features in OS MasterMap Topography Layer, it is necessary to look at the accuracy of position attributes of the topographic line features, which form the boundary of the area feature polygon. Please see the User Guide for further details.

Each and every feature within OS MasterMap® has the TOID® (TOpographic IDentifier) as a unique reference. This makes it possible to identify any single feature within the dataset without any ambiguity.

Current referencing systems, such as coordinates and addresses, are subject to interpretation between users. This creates fuzzy matching issues between organisations wanting to exchange data; for example, will the building St James House be picked up in another system where it is spelt St James' House?

TOIDs will stay the same throughout the life cycle of each feature. In other words, if a feature changes, the TOID will stay the same as long as it is deemed to be the same feature; for example, a house having an extension built is still the same house and fulfils the same purpose. This provides a continuity of reference even though a feature may undergo changes.

This makes the TOID a robust referencing system that customers can use alongside, or instead of, their own referencing systems and can link to their own information about the feature.

Having unique references is essential to making the most out of storing data within a relational database management system, as these rely on unique referencing to be able to store, sort, manage, query and retrieve data efficiently.

Building Height Attribute data will be updated every six months, with releases scheduled in April and October. See the latest release note for more.

Building Height Attribute data is available in CSV file format only.

Building Height Attribute has been released as a separate CSV file so there is no requirement to change the Topography Layer schema.