There are several options available to customers when ordering data in GML format that provide additional metadata or aid data management.

Chunk files

To make the management of large areas easier, data is split into chunks, each of which covers a nominal square area. Data can also be supplied as part of a squared area to complete a predefined area as part of a nominated file size. Two types of chunks are available: geographic and non-geographic chunks. Chunk boundaries are imposed purely for the purpose of dividing large supply areas into pieces of manageable size in a geographically meaningful way. Both Full Supply and updates (whether COU or Full Resupply) are chunked.

Geographic chunking option

As OS MasterMap Topography Layer data is seamless, GML files containing large areas could be very data intensive. In order to provide files of a manageable size, data supplies are divided into chunks of user-specified size, each of which is supplied in a separate GML file. The figure below illustrates how geographic chunks work:

The process of chunking has the following steps:

1. The user submits an AOI by either drawing an AOI or uploading a pre-defined AOI to the OS Data Hub platform (or OS Orders).

2. Both online ordering systems create a grid covering the entire area based on the user-specified size (for example, 25km²).

3. Each square within the grid forms a chunk file.

4. Each feature that intersects that square goes into the chunk file.

In the case shown in the image above, 10 chunks have been created. The central chunk is a complete grid square; the other chunks are partly bounded by the data selection polygon. The upper-left square shows the effect when the data selection polygon crosses a grid square twice – i.e. two or more separate chunks are created.

A consequence of chunking is that some features are supplied in more than one chunk. Systems reading OS MasterMap Topography Layer data must identify and provide the option to remove these duplicated features.

If a chunk contains no information relating to a user’s selected themes, then it is not supplied.

Chunks cannot be treated as persistent data management units; as it is a floating grid, the origin of the chunking grid may differ between orders, particularly if the contract area changes or if a different chunk size is ordered.

Supply of OS MasterMap Topography Layer features in chunk files

The packaging of a seamless dataset into chunks means that where a feature lies across or touches the boundary of several chunks, it is supplied in all the related chunks. This is because an individual feature is the smallest unit within the OS MasterMap Topography Layer, and it cannot be physically split into two or more parts.

When a polygon falls across a chunk edge, but its bounding line(s) lie outside, it may not be included in that chunk. It will be included in the adjacent chunk, unless the polygon is at the edge of the contract area, in which case, the line will not be supplied at all.

When a polygon changes so that it no longer falls in the same chunk, for instance, when an OS MasterMap Topography Layer feature used to lie partly inside a chunk and instead is now reduced in size so it is wholly within an adjacent chunk, it is reported as a deleted feature (a Delete) in one chunk and as a modified feature (new version, known as an Update) in the adjacent chunk. This is shown in the diagram below.

It is possible for OS MasterMap Topography Layer features with point geometry to be included in multiple adjacent chunk files. This is because the query used to populate a chunk file includes all features that touch its boundary, and this boundary is shared with adjacent chunks. Therefore, loading software must be able to identify and remove duplicate point features across multiple files in the same way as features represented by lines and polygon geometries.

Non-geographic chunking option

This supply format delivers the files in a fixed nominal size, as opposed to a given geographic area depending on user preference.

Unlike in geographic chunking, each feature in non-geographic chunking appears in only one chunk file. It is possible for features from various geographic locations to appear in a single file, and for adjacent features to appear in different files. Non-geographic chunk files are designed for use as a set to load spatial databases but can be used in a file format if all chunks are translated or imported into the system at the same time.

It is not possible to tell in which file a particular feature will be found before reading the files. With non-geographic chunks, there are no duplicate features lying across chunk edges; this speeds up the translation process.

The features shown in red below can end up in the same non-geographic chunk even though they are not adjacent to each other.

Feature Validation Dataset

The Feature Validation Dataset (FVDS) is a set of files that can optionally be supplied with either a Full Supply COU or an AOI COU of an OS MasterMap Topography Layer order in GML format. The FVDS allows a user to validate that the data holding contains the correct set of features after loading. It does this by reporting on all the data it expects to find in the holding after the application of the supply, not just what is contained in the supply.

The FVDS is intended to be used for periodic checks on data holdings maintained by a COU regime. It is not necessary for users to order it with every supply, as processing it will slow the translating process. It can also be used to check that an initial supply of OS MasterMap Topography Layer data has been correctly loaded. The FVDS can be used with both geographic and non-geographic chunk file options.

The FVDS is itself divided into files on a non-geographic basis, using a 10MB nominal file size.

The FVDS is a comma-separated value (.csv) text file format that gives the TOID, version number and version date of every feature that should exist in the current data holding, based on the polygon extent,

themes, polygon format and extraction date of the current order. Each .csv file is compressed to a .gz file using the same compression algorithm used for OS MasterMap Topography Layer GML files.

GML summary file

An order summary file in GML format will be supplied with all OS MasterMap Topography Layer orders, containing the order information specified by the user. This information includes:

• The order number

• Query extent polygon(s) of the order

• The order type: Full Supply or COU (for COU orders, the change since date will be included)

• The themes requested

• The chunk type: Non-geographic or geographic

• The chunk size (in MB for non-geographic chunks; in km² for geographic chunks)

System requirements

OS MasterMap Topography Layer data is designed for use as a digital map within GIS and database systems.

For details of Ordnance Survey’s Licensed Partners who can incorporate OS MasterMap Topography Layer in their systems, please see the Our Partners page on the OS website.

Ordnance Survey does not recommend either suppliers or software products. The most appropriate system will depend on many factors, such as the amount of data being taken, resources available within the organisation, the existing and planned information technology infrastructure or, last but by no means least, the applications in which OS MasterMap Topography Layer is to be used.

However, as a minimum, the following elements will be required in any system:

• A means of reading the data in its native format, or translating the data into a file format, or storing the data in a database.

• A means of storing and distributing the data, perhaps in a database or through a web-based service.

• A way of visualising and querying the data, typically a GIS.

There is more information on handling OS MasterMap Topography Layer in product supply.

Available formats for the product

• GeoPackage

• Vector tiles (MBTiles)

• Geography Markup Language (GML) 2.1.2

Coverage

Coverage is Great Britain (GB).

File size

File sizes vary greatly depending on tile size and the quantity of features present in a tile:

• GeoPackage 25km² tile range: Varies (dependent on AOI selection)

• Vector tile GB Supply: Approx. 20GB

• GML 25km² tile range: Varies (dependent on AOI selection)

• GML GB Supply: Approx. 47GB

Product update schedule

OS MasterMap Topography Layer is updated every 6 weeks. Information about the current product version, and information about future and past releases, can be found on the OS MasterMap refresh dates page of the OS website.

OS MasterMap Topography Layer incorporates a web-based ordering system that allows users to order their initial data supply and any updates, obtain price estimates and view details of their holdings on demand. The product is supplied as an online download. You can download data in its various formats from the OS Data Hub.

Please note that some formats are only available via OS Orders, Ordnance Survey's legacy ordering system; these formats are indicated in the following table:

* Feature Validation Dataset (FVDS) and Change-Only Update (COU) available.

† Only available via OS Orders.

There are two main supply options available for OS MasterMap Topography Layer: Full Supply or COU.

Initial Supply refers to the first order of OS MasterMap Topography Layer that a user makes, which will contain all features for all the layers selected for the area covered by the order. Upon Initial Supply, users can select whether they would like to receive future data updates as COU (user receives only the tiles/features that have changed since the last update) or a Full Supply (user receives all tiles/features regardless of whether they have changed since the last product update). COU data contains new and updated versions of features (known as Inserts and Updates, respectively), and information about departed features (known as Deletes); any feature within the area covered by the order that has not undergone any change will not be supplied.

The advantage of taking a COU Supply rather than a Full Supply is that, if applied regularly, the amount of data that must be processed and loaded is much smaller with a COU Supply. Users may request updates of the latest changes in their AOI, at any time, using the online service. Updates can either be for specified AOIs or GB wide.

The underlying database for OS MasterMap Topography Layer is live and undergoes continuous revision. When a user orders COU, a ‘change since’ date is specified, and all features that have changed since midnight on the date specified are supplied. This will normally be the date the data was last extracted from the Ordnance Survey main database but could be a previous date. The last extraction date can be found on the along with the update history and release notes for the product.

To be able to resolve changes to the data holding, the system used to translate or load the data must check the TOID and version of every feature in the update against the current data holding to determine whether it should be loaded, and if so, what existing feature(s) it replaces. This makes it possible to request and load COU with a date preceding the last data supply date without damaging the data holding. This process can be used to correct a data holding if inconsistencies have occurred due to partially loaded or non-sequential COU, by ordering a single COU with a change since date that precedes the problem updates.

Managed Great Britain Sets

For users taking a Full Great Britain (GB) Supply of OS MasterMap Topography Layer, there is a Managed Great Britain Sets (MGBS) service. The MGBS service is available for GML or vector tiles formats only (full GB coverage of OS MasterMap Topography Layer in GeoPackage format as a single AOI will be too large for the majority of GIS to handle).

OS Data Hub users will be automatically notified (via email) when their Full Supply or COU MGBS is available from their 'Data Package' area on the OS Data Hub, in accordance with product refresh dates as stated on the .

OS Orders users will automatically receive their updates (Full Supply or COU) on DVD or hard drive, either quarterly or every six weeks (whichever option was selected at initial order).

MGBS users and Ordnance Survey Licensed Partners that take GB coverage can benefit from:

• Data arriving faster and in a more predictable and timely manner

• Seeing the same version of features as other organisations

• Easier data management

• A Feature Validation Dataset (FVDS) with all COU orders or with Full Supplies on request (GML format only)