OS MasterMap

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OS MasterMap is Ordnance Survey's flagship digital product, launched in November 2001. It is a database that records every fixed feature of Great Britain larger than a few metres in one continuous digital map. Every feature is given a unique TOID (TOpographical IDentifier), a simple identifier that includes no semantic information. Typically each TOID is associated with a polygon that represents the area on the ground that the feature covers, in National Grid coordinates. MasterMap is offered in themed "layers", for example a road layer and a building layer, each linked to a number of TOIDs. Pricing of licenses to MasterMap data depends on: the total area requested, the layers licensed, the number of TOIDs in the layers, the period in years of the data usage.

MasterMap can be used to generate maps for a vast array of purposes. Although the scale on a digital map is much more flexible than a paper map, one can print out maps from MasterMap data with detail equivalent to a traditional 1:1250 paper map.

The OS claims that MasterMap data is never more than 6 months out of date, thanks to continuous review. The scale and detail of this mapping project is so far unique. Around 440 million TOIDs have so far been assigned, and the database stands at 600 gigabytes in size. MasterMap is currently (August 2005) at version 6.

The OS is encouraging users of its old Land-Line data to migrate to MasterMap.

[edit] Delivery of the data

OS MasterMap data is in GML format. It is usually delivered as files compressed with gzip (giving them an extension 'gz').

Recently the OS have been trialing delivery of MasterMap using WFS and WMS, in accordance with the Open Geospatial Consortium. This trial may even end up with automatic updates using WFS-T.

[edit] Custodianship of the data

Although branded like a commercial product, OS MasterMap is in fact the nation's most complete, accurate and up-to-date geographic record. By moving to purely digital media for mapping, Ordnance Survey brings upon itself the responsibility of being the sole mandatory custodian of Great Britain's official maps. Previously, by virtue of being a publication on paper, a copy of every edition of every paper map published by the OS would have been deposited, by law, with each of the six legal deposit libraries in the UK. There is no such law in place for digital maps. The data, like any other, is susceptible to all the problems of digital archives and computer backups, such as data corruption and the policy of how often to take snapshots of the data (i.e. should every edit be recorded?). It is unclear if one will be able in 50 years' time to get a snapshot of today's data. Indeed, the OS website (October 2005) suggests not: "Ordnance Survey does not hold archives or historic views of the data"[1]. The map librarian at Cambridge University Library, one of the deposit libraries, reports (September 2005) an initial reluctance by the OS to supply map data to the library, and compliance only on strict conditions of access. This compliance was voluntary, unenforceable by law. Subsequently, the OS themselves requested access when their own data for a particular period and place appeared to be lost.