This product may be used on a wide range of hardware platforms (provided sufficient memory and storage facilities are available), varying from tablets or computers using GIS or computer-aided design (CAD) to mainframe computers with specialised translators and applications. Please see Coverage and file sizes on the product supply page for more information. Your system supplier will be able to advise on your requirements.
OS Terrain 5 is supplied as inert data in a variety of formats and does not include software for data manipulation.
GML is an open standard format, and the data may need to be translated into the appropriate format for use within a GIS application. A wide range of GIS software can read the GML contour data without translation.
OS Terrain 5 is available as:
5m grid in ASCII grid and GML 3.2.1 (simple features profile – level 0).
5m contours and spot heights in Esri shapefile or GML 3.2.1 (simple features profile – level 0).
OS Terrain 5 data is provided as a full set of Great Britain in tiles and is available as a download only from the OS Data Hub or OS Orders. The national dataset is supplied as 5km-by-5km tiles of data. There are 10,579 data tiles in the product representing each 5km-by-5km tile grid square. Please note that there are slightly fewer data tiles in the contour set (10,572) due to the lack of real-world changes in height in those areas. There are metadata files for these tiles.
It is recommended that a download manager is used to extract the data as this additional functionality will be able to automate the process and organise the data folders as desired. There are many commercial and open-source download clients available to help manage the data.
OS Terrain 5 is full national coverage of Great Britain. Terrain 5 is derived from the same source data as our large-scale revision programme.
Data will be compressed using the zip compression method and is not encrypted. The respective file sizes for a full national supply of OS Terrain 5, in each format, is approximately:
Grid: 16GB compressed
Contours (GML): 2.6GB compressed
Contours (shapefile): 2.5GB compressed
OS Terrain 5 is supplied to customers quarterly as a full tile resupply in January, April, July, and October, incorporating any updates made by the revision programme.
This overview introduces OS Terrain 5 and gives context for all users – highlighting key features, providing examples of uses, and listing details such as product accuracy, supply formats, etc.
OS Terrain is the name given to Ordnance Survey’s height products. The two products available, OS Terrain 5 and OS Terrain 50, provide detailed three-dimensional digital terrain models (DTMs) of Great Britain. A DTM primarily defines the ground surface, having removed all protruding features (such as buildings and trees) elevated above the bare earth. The main difference between the two products is their level of resolution:
OS Terrain 5 is a mid-resolution DTM, designed to be interoperable with our large-scale data.
OS Terrain 50 is a lower-resolution DTM product, designed for landscape visualisation and analysis over large areas. It is an OS OpenData product and, as such, it is free to view, download and use for commercial, educational, and personal purposes.
OS Terrain 5 is published as both grid and contour data, each in a variety of formats. Both data types are created from the same source data and are supplied as 5km-by-5km tiles.
OS Terrain 5 grid: a grid of heighted points with regular 5m post spacing.
OS Terrain 5 contours: a contour dataset of 5m interval standard contour polylines, which includes index contours at 25m intervals, mean high and low water boundaries and spot heights.
The product is maintained by our integrated 3 to 5-year flying programme and is synchronised with our other product updates.
There is specific modelling of significant features, particularly networks.
DTM grid data is available in ASCII (American Standard Code for Information Interchange) grid and Geography Markup Language (GML) 3.2.1, using Open Geospatial Consortium (OGC) Simple Features Profile (level 0).
DTM contour data is available in GML 3.2.1, and Esri shapefile.
There is full coverage of Great Britain.
It is designed to work with Ordnance Survey’s large-scale data.
Supplied as grid and contours with spot heights.
The product is updated quarterly.
It is based on the INSPIRE elevation specification.
OS Terrain 5 provides an ideal base to enable the third dimension for other data within an appropriate geographical information system (GIS). The height data has been created from a source that is also used to update our large-scale data products.
OS Terrain 5 can be used in conjunction with various Ordnance Survey digital products for analytical, modelling, planning and visual purposes. These uses enable customers to undertake the following:
Asset/site management
Environmental analysis
Line of sight planning
Viewshed modelling
Landscape visualisation and fly-through sequences
Planning and development
Signal propagation
Wind farm location planning
Flood risk assessment
Foundation for 3D modelling
OS Terrain 5 has been measured against with GPS points in a range of sample areas to provide a root mean square error (RMSE) value, being the average of the mean height points in each geographic area: urban and major communication routes, rural, and mountain and moorland.
Urban and major communication routes 1.5m RMSE
Rural 2.5m RMSE
Mountain and moorland 2.5m RMSE
OS Terrain 5 is designed to be INSPIRE-ready. Ordnance Survey is a leading member of the UK Location Programme, which is charged with delivering INSPIRE, a directive that applies to all member states and aims to enable more joined-up data across public bodies for environmental applications.
At the time of OS Terrain product development, the INSPIRE elevation specification had not been finalised. The data structures of OS Terrain products, and the details of the GML encoding, have been based on the draft INSPIRE specification. For more information about INSPIRE and UK Location, please view the INSPIRE web pages.
This page describes the structure of the data in grid and contour forms.
The height data is presented as a raster dataset of height values, which are calculated at the centre of the pixel. Coordinate reference systems for DTMs may be used to calculate the DTM origin and coordinates of individual posts.
Due to local tidal conditions, the height of the mean high and low water mark varies continuously around the coast of Britain. The mean high and low water lines have been derived from our large-scale mapping and assigned constant height values, based on the average for each tile. This average value has been determined from local tide tables. The mean high and low water lines were used as heighted breaklines when creating the grid to ensure the grid product is consistent with the contour product. This means that there may be a small discrete step in the height of tidal water between adjacent tiles. For areas of permanent tidal water, the height of the mean low water has been extended out to the tile edge to ensure that the tile is complete. Heights in the foreshore area are interpolated between the mean high and low water heights.
The contours are presented as polyline and spot height features. The contour lines have been divided into tiles for product supply. The contour values can be viewed and analysed in a GIS.
The terms used for the feature types are drawn from the INSPIRE elevation specification. The draft INSPIRE elevation specification requires height values to be held in an attribute called ‘propertyValue’, which has been implemented in the new OS Terrain contour GML. In the shapefile format, the GML feature types and attributes have been followed but with the 10-character limit on shapefile fields, ‘propertyValue’ is abbreviated to ‘PROP_VALUE’.
The contours are named ‘master’ and ‘ordinary’, which are equivalent to ‘index’ and ‘standard’ contours in the withdrawn Ordnance Survey height product, Land-Form PROFILE.
In the contour products, the mean high and mean low water lines have been derived from our large-scale mapping and assigned constant height values, based on the average for each tile. This average value has been determined from local tide tables. Inevitably, this means that there is a small discrete step between adjacent tiles. There are no contours supplied between the mean high and low water lines.
The ‘LandWaterBoundary’ feature type has an attribute of ‘waterLevelCategory’, which has four possible sub-values: ‘meanHighWater’ and ‘meanLowWater’ or, for Scotland, ‘meanHighWaterSprings’ and ‘meanLowWaterSprings’. This attribution allows their display parameters to be changed to show individual features, as desired.
These have been created using an algorithm that selects the highest source data point (masspoint) within every enclosed contour that has a difference in height of at least 1m.
As they are from an interpolated surface of the real world, they cannot be guaranteed as summits or highest points of the feature. The GML data model lists the spot height sub-value, ‘spotHeightType’, which allows the potential for formSpot, generic, mountainPass or summit to be used in future releases of the product.
Currently, all spot height features are attributed as ‘generic’.
It is possible that there will be some instances of spot heights recording lower height values than the enclosing contour, but it is likely that these are the result of genuine depressions.
The source DTM for OS Terrain products is captured as a triangulated irregular network (TIN) by editing with mass points and breaklines and/or automated techniques within a photogrammetric environment.
The TIN is a superior model for three-dimensional data as it uses triangles, which can retain the edges of features more accurately than a grid, for example.
The source data capture is subject to demanding rules defined by the height capture specification. Particular attention is paid to communication routes and features significant to height applications. This section describes some of the key capture requirements from the detailed capture specification that we endeavour to achieve in the source data.
The grid and contour products are both interpolated from this source TIN model.
The minimum coverage of the data extends out to the low water mark, defined by Hydrographic Office tables with a height value for each 5km-by-5km tile. For England and Wales, the low water mark is mean low water (MLW) and for Scotland, mean low water (springs) (MLW(S)).
All land wholly within inland water bodies that is represented by topographic area features is captured according to the positional accuracy requirements of the area. The minimum requirement is to capture the outer edge of the feature. The surrounding water will remain flat.
Any other land within inland water bodies captured by automated processes will be removed from the data.
The z values of the source TIN data must meet positional accuracy requirements according to their geographic location. The terrain has been divided into three classifications – urban and major communication routes, rural, and mountain and moorland – to ensure that modelling reflects customer requirements. The accuracy of the height value above Ordnance Datum Newlyn must achieve the RMSE set for each area, which are not the same as the stated product accuracy.
The height of the bare earth surface is recorded as a series of points with three-dimensional coordinates.
The X and Y coordinates are eastings and northings in OSGB36; the Z coordinate is height in metres relative to the datum for the area. Most areas will record a height relative to Ordnance Datum Newlyn. For a small number of offshore islands, a local datum has been used.
The bare earth surface excludes buildings, supported structures, and vegetation. Structures that form an obstruction at ground level – such as dams, breakwaters, and groynes (wide enough to affect the positional accuracy requirements), bridge revetments and earthworks – are considered to be part of the bare earth surface. Only permanent terrain features are modelled.
Underground and overhead features are, by definition, not the ground surface and are thus not included in a DTM. Underground features are those that are obscured and require excavation to construct.
Underground features are not recorded, and overhead features are removed from the data.
The DTM will be free of spikes and wells that do not reflect the real-world terrain. A surface that is smooth; that is, one that consists of a regular plane (which may be angled); for example, a road carriageway or railway trackbed, will also appear smooth in the data.
Most data will present without visible tile edges or discernible height differences between tiles. In places, there may be small edges present or a difference in feature modelling between new and older content. There will also be small edges in tidal areas due to local tidal differences.
Supported structures include bridges, viaducts, jetties or piers on legs, cranes, elevated buildings, and so on.
Supported structures are removed from the data where the structure departs from the bare earth surface and an air gap exists. All supported structures will be removed from the data.
Areas of vegetation, such as hedgerows and trees are removed to ensure the bare earth surface is correctly recorded.
Locations with a vertical change in height, or overhang, have the height of the top of the feature recorded at the correct planimetric location according to the positional accuracy requirements.
The height at the lowest point of the vertical feature is recorded according to positional accuracy requirements of the feature but offset from its real-world planimetric position to ensure that there is only one z value present in the same location.
Major communication routes are major road and rail networks identified in our core database.
The limits of a road carriageway or railway track bed are modelled to ensure that the route reflects its real-world shape. Modelling is required for changes in height to meet the positional accuracy requirements, to smooth the surface, and to remove extraneous features such as road furniture and bridges. Any associated slopes and embankments along the length of the route are also modelled.
In all other cases, the surface must be smooth, flat (not necessarily horizontal) and free from undulations
The outer limits, shape and depth of mineral extraction and landfill sites are captured to meet positional accuracy requirements. Temporary features that do not represent the terrain at the time of capture, for example spoil heaps, are removed from the data.
To respond to the Flood and Water Management Act 2010, the extent of all flat-water bodies greater than 0.7ha in area (that is, greater than 7,000 cubic metres capacity) will have their limits captured to ensure that the presence of the water body can be inferred from the data.
The height of the water recorded is that at the lowest height of the surrounding data. The surface of the water will be flat.
The following measurables are used to determine the accuracy and coverage of modelling in the source TIN data.
Any height value extracted from any point within the coverage and above the high-water mark will meet the positional accuracy requirements.
The DTM will be free of spikes and reflect the general texture of the terrain.
Measurable
The absence of spikes or wells in the data that cause a false height value on a data point or interpolated surface.
A surface smooth in the real world will appear smooth in the data.
Major communication routes will be smooth longitudinally, with no ‘steps’ in the data.
Conformity
There must be no spikes or wells in the data larger than the value required to meet the positional accuracy requirements of the geographic area.
Data added to model specific features will reflect the correct relationship to itself and immediately surrounding data.
All features with parallel limits that require modelling will have parallel data points recorded in the data. The relative heights of features in proximity will be in sympathy.
The coordinate position of any point on the surface as represented by the TIN data compared to the true elevation of the same point.
The RMSE of a selection of points from the real world compared to the data will be within the values stated.
No single point will exceed the 99% confidence level.
OS Terrain 5 is supplied as a whole set of Great Britain divided into 5km-by-5km tiles. These tiles are identified by quoting the National Grid reference of the south-west corner of the area they cover.
Ordnance Survey divides Great Britain into squares 100km-by-100km. Each of these squares has a unique two-letter reference, for example, TG in the diagram below.
To describe a 10km-by-10km tile, first add a two-digit reference to the 100km-by- 100km square reference, with the easting first followed by the northing: for example, TG23.
Then identify which quadrant of that grid square is required and add SW, SE, NW, or NE to the reference, for example, TG23SW.
For additional information on how to use the National Grid, visit the Ordnance Survey website.
