Scale describes the ratio in size between something in the real world and its representation on a map. In other words, if a map has a scale of 1:50 000 then a distance of 1 cm on the map represents 50 000cm (500 m) in the real world.

The main scales used by us for surveying are 1:1250, 1:2500 and 1:10 000. These are referred to as large scale maps, which is when features in the real world are portrayed larger on the map.

We produce a number of mapping products at other scales, such as 1:25 000 scale and 1:50 000 scale for example. These are generally referred to as small scale. On these small scale maps, due to scale restrictions, some features are simplified or generalised. For example, at these scales we do not show the shapes of invidual houses. Without generalisation, small maps would have too much detail to be able to read.

• 1:1250 scale is used for surveying cities and towns (urban areas) and is the most accurate we use.
• 1:2500 scale is used for smaller towns and villages (rural areas).
• 1:10 000 scale is used for remote areas, mountain and moorland. Generalisation of features may occur at this scale.

To find out the scales of individual products, please refer to our product pages.

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We define accuracy in three ways:

Absolute accuracy is the measure which indicates how closely the coordinates of a point in our map data agree with the ‘true’ National Grid coordinates of the same point on the ground.

Relative accuracy is a measure of how much detail is drawn on the map and how realistically the objects relative positions are depicted.

Geometric fidelity is a way to ensure any real-world alignment or shape is accurately reflected in the data to the required specification. For example, detail that is square on the ground must be represented as square in the data and shapes must be accurate.

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

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

The diagram below shows this in more practical terms.

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As the landscape is constantly changing, we are unable to survey and map every change as it occurs. We work to guidelines that determine what should be mapped and when, based upon the greatest benefit to all of our customers.

As an example, if you have just built an extension on your house and it does not show up on our maps, this is because we consider this type of change as a low priority, compared with mapping the construction of a new school building.

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We are updating our central database every day by both field survey (continuous revision) and from aerial photography (cyclic revision). The data has to go through a number of different databases and production processes before it can be released in our products. Due to the differing scales of our products and the level of detail shown, some products need to be updated more frequently than others (as they show the most detailed and hence the most changes on the ground).

For our most detailed products (OS MasterMap^® Topography Layer), our Basic Scale Revision policy sets out which types of changes on the ground will be updated and when.

The table below details the frequency of published product updates for all of our digital products.

Our paper maps, OS Explorer and OS Landranger, are generally revised on a maximum five year revision cycle; that is, each map should be revised for significant changes at least once every five years.

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The format of the file describes in which way the geographical features have been saved. If you wish to use data that we save in a format that is not supported by your GIS application, you will need to convert the data files into a format that your application does support (please contact the vendor for further information).

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Each of our products and services pages provide you with detailed information, such as how it can be used, technical information and help with frequently asked questions.

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To get the most out of our data, you will need a Geographic Information System (GIS). Since we do not develop GIS software ourselves, we work closely with our Partners to ensure that a range of solutions are available to help you.

Some products like OS MasterMap layers that are supplied in GML format as .gz zip files may also need additional translation tools or software to be read by your GIS. Please check with your GIS provider.

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Geo-referencing is the intelligence that connects/positions each tile of mapping within a geographic information system (GIS), to join up each adjacent square.

Geo-referencing files allow tiles of map data to be located in their correct geographical position when loaded into a GIS. Our raster data products when supplied to you are not geo-referenced, however we can provide geo-referencing files to work with all scales of our raster products.

Please note that OS VectorMap Local Raster versions are supplied as GeoTIFF with geographic (or cartographic) data embedded as tags within the TIFF. Separate geo-referencing files are not required to position OS VectorMap Local - Raster versions in a GIS.

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We have three different types of web map services:

1. OS OpenSpace is a free service which uses an application programming interface (API) to embed our maps onto your website.
2. OS OpenSpace Pro is an extension of OS OpenSpace, which gives you access to extra map layers and supports commercialisation of your website application.
3. OS OnDemand enables you to use our maps within your organisation as a managed service. It is particularly suited to those companies running GIS departments and wishing to serve mapping to many users across the organisation.

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Our Global Navigation Satellite System (GNSS) correction network is call OS Net^®. We have a network of around 110 permanent GNSS stations around the country. From the observations made by these stations, errors can be calculated to provide a range of correction services.

OS Net as a service is not available commercially. Our Partners, however, use this network data to develop and provide a basket of real-time and post-process GNSS correction services to customers within a wide range of markets.

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In 2006, we ran a programme to increase the accuracy of our products within rural areas. This has been completed, but we still have a PAI summary and other useful documents in case you need to implement any of the changes arising from this work. Please see the link files page to find out how to request this archive data.

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