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OS MasterMap FAQs

Where can I get a historic view of OS MasterMap?

Ordnance Survey does not hold archives or historic views of OS MasterMap® data.

If you have been directly licensed by Ordnance Survey to use OS MasterMap data over a continuous period of time, and have accumulated data during this period, you may hold and use it to provide historic views so long as you continue to remain directly licensed for the data by Ordnance Survey.

If you are not a licensee and wish to view and undertake limited copying of historic versions of OS MasterMap data, then you may do so at one of the six Legal Deposit Libraries that hold annual 'snapshots' of large-scale Ordnance Survey data, which Ordnance Survey voluntarily deposits by Agreement with these Libraries. The data may only be accessed within the premises of one of these Libraries, and views and prints made from the data may only be used for purposes included within the exceptions to the Copyright Designs and Patents Act 1988.

The six Legal Deposit Libraries are: The British Library; The Bodleian Library, Oxford; Cambridge University Library; The National Libraries of Scotland and Wales; and the Trinity College Library, Dublin.

How many unique feature types/feature codes are in OS MasterMap?

There are seven feature types within OS MasterMap Topography Layer:

  • Topographic point
  • Topographic line
  • Topographic area
  • Boundary line
  • Cartographic symbol
  • Cartographic text

There are 92 feature codes, which are made up of three attributes – the feature type, descriptive group and descriptive term.

How often is OS MasterMap updated?

The survey data that is used to create OS MasterMap is captured on a daily basis by both field survey and from aerial photography. The data has to go through a number of different databases and production processes before it can be released to the customer. This means that the database from which customers are supplied is actually refreshed every six weeks.

How can I notify you of any errors or omissions in OS MasterMap?

We are always pleased to accept information from our customers that will help us to improve the quality of our data. Please forward, with as many details as possible including a postcode, co-ordinates or TOID reference, errors or omissions to customerservices@ordnancesurvey.co.uk or via the post to Ordnance Survey, Adanac Drive, Southampton, SO16 0AS or telephone 08456 05 05 05.

What is a feature validation dataset (FVDS)?

The FVDS enables OS MasterMap users to verify their data holding. It is a list of TOIDs with their corresponding version number and version date. There is an option on the OS MasterMap online service to take a FVDS for an order.

What is the difference between GZ and GML format?

GML data is supplied in a compressed format called GZIP, which is denoted by the '.gz' filename suffix. Compression using the GZIP algorithm allows OS MasterMap data volumes to be reduced by a ratio of between 20:1 and 30:1.

My system reads NTF directly. Do I need a translator for all layers?

You will need a GML translator for all layers apart from OS MasterMap Imagery Layer. Please contact your system supplier for further information.

When I load OS MasterMap, all I see is a series of lines. Why is this?

Although some systems are able to directly read OS MasterMap data in its native GML format, when read in this way the data is likely to be rendered in an unstructured way without any meaningful symbology. Please contact your system supplier for further information on displaying the data within your system.

Do we have to buy decompression software to decompress OS MasterMap?

There are a number of commercially available tools such as WinZip® (which is already built into many Windows® 98, Windows NT® and Windows 2000 systems) and ZipMagic together with freeware such as PowerArchiver 6.1, which can be used to uncompress GZ format files. For more information, see the gzip website. It is important to note that although decompressing GZ format files will let you examine the GML, typically you will need to translate the GML to allow it to be used within a GIS or CAD system.

I can't find data in the directory folders I would expect to? Is it missing?

Because of the way OS MasterMap GML files are named in an order, it can appear at first glance that only certain areas of data have been supplied.

The polygon that you submit within order for OS MasterMap data can create a situation where the data is supplied in a different folder to the one you may have expected.

This is due to the use of a ‘floating chunk grid’. This is specific to the order polygon and does not use a regular pattern of fixed tiles. This chunking grid has been used to ensure data is supplied with the minimum amount of separate files.

The OS MasterMap ‘chunks’ will be created using the size of chunk you have selected as part of the ordering process.

For example, if you order data for SU7325 and SU7425 the folder on the OS MasterMap order will be named to the SW corner of the 5km x 5km chunk that contains those tiles. In this case, the single file name will include the National Grid reference of SU7325 but will also contain the data for SU7425. The next GML folder if applicable to the order would then have SU7825 as part of its file name, which will be 5km on from the first chunk.

Therefore, if you cannot find the data in the folders you are expecting to find them in, please open other folders in the correct National Grid sequence applicable to your chunk size to check the contents.

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