OS MasterMap Topography Layer support


The Topography Layer is the most detailed, current and comprehensive map dataset of Great Britain. You can reference your data with it to any real-world feature, helping you to manage your organisation's assets more effectively.

Descriptive terms

Enhanced descriptive terms for land, water and structures are being released. Descriptive terms let you interrogate, count, and use more geographical features for large-scale analysis.

Descriptive terms information and release plan

Schema 9 and data fulfilment

The Schema 9 upgrade was released in March 2017. We also went live with an upgraded data fulfilment system.

Schema 9 and fulfilment system information

Documents & links

Topography Layer

Building Height Attribute

Sites Layer

How often is the OS MasterMap Topography Layer updated?

All OS MasterMap® datasets are created from a single master dataset, which is refreshed regularly. Currently, the refresh period for Topography Layer supply is every six weeks.

OS MasterMap Topography Layer works on the principle that you will take an initial supply of all data in your area of interest and then update the data by taking change-only update (COU).

COU brings your dataset up to date with the most recent OS MasterMap data available from Ordnance Survey.

The update of real world features within OS MasterMap Topography Layer depends on which category of change the feature falls in, as described in our Basic scale revision pollcy.

How can I find out if a certain type of real-world object is represented?

Please see the real-world object catalogue (12 Mb PDF), 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.

What scale is OS MasterMap Topography Layer data?

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.

How accurate is OS MasterMap Topography Layer?

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.

Original survey scale99% confidence level95% confidence levelRMSE*
Absolute Accuracy 0.9 m 0.8 m 0.5 m
Relative Accuracy +/- 1.1 m (up to 60 m) +/- 0.9 m (up to 60 m) +/- 0.5 m (up to 60 m)
Absolute Accuracy 2.4 m 1.9 m 1.1 m
Relative Accuracy +/- 2.5 m (up to 100 m) +/- 1.9 m (up to 100 m) +/- 1.0 (up to 100 m)
1:10 000
Absolute Accuracy 8.8 m 7.1 m 4.1 m
Relative Accuracy +/- 10.1 m (up to 500 m) +/- 7.7 m (up to 500 m) +/- 4.0 m (up to 500 m)

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

What are OS MasterMap Topography Layer themes?

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 (PDF).

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

Do Topography Layer boundaries represent legal property extents?

No, OS MasterMap Topography Layer doesn't show information on land and property ownership.

How do I determine the captured accuracy of a feature (capture scale)?

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 (PDF) for further information.

How does OS distinguish between attribute change and physical change?

OS provides the answer to this question in the ‘Reason for change’ attribute:

If the attribute of the feature have changed the ‘Reason for change’ attribute would be Reclassified.

If the geometry of the feature changed (the feature has been edited by an operator) the ‘Reason for change’ attribute would be Modified.

When both attribute and geometry have changed, the ‘Reason for change’ would be also Modified.

What are the advantages of TOIDs as a referencing system?

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.

Where can I find details of the Topography Layer upgrade?

Please see our guide to the changes, where you can learn about the new content, styling options and more.

How often will BHA be updated?

Building Height Attribute data will be updated every six months, with releases scheduled in April and October.

What file format is BHA available in?

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

Why is Building Height Attribute not included as part of Topography Layer?

Building Height Attribute has been released as a separate CSV file so there is no requirement to change the Topography Layer schema. For information on how to join BHA to Topography Layer data, please see our getting started guide (PDF).

Why do some of the 5kmĀ² tiles contain no data?

For the Beta release, it's possible that some of the coverage tiles will contain no data. This occurs when the data within that tile has been discarded as corrupt and has failed validation checks. Over time, as the product is updated and coverage improves, this will become less likely. See our BHA tiles CSV for information about which tiles currently contain no data.

Why does the product contain building heights from the Alpha release?

We've included data from our 2014 Alpha release where there are areas that do not have more up-to-date Digital Surface Model (DSM) and Digital Terrain Model (DTM) source data.

Also, areas of predominantly Beta Building Height data, buildings that have missing BHA values, or have clearly been generated from corrupt DTM or DSM data, have been replaced with existing data from the 2014 Alpha product. This means that within any area there can be a combination of buildings with heights derived from the Alpha and Beta products. These areas will be updated over time, as new source data is created.