- What is OS Terrain?
OS Terrain is the name given to Ordnance Survey’s new range of height products, These are three-dimensional models of the bare earth surface known as Digital Terrain Models (DTMs). The range will consist of two products:
- OS Terrain 5 - A mid-resolution DTM product, designed to be interoperable with our large-scale data
- OS Terrain 50 - A lower-resolution DTM product, designed for landscape visualisation and analysis over large areas. The grid is now available through OS OpenData™.
- What are the benefits of OS Terrain 5?
Customers who are using other Ordnance Survey data products will not find another height product in the market that offers this level of consistency, coverage and currency with our topographic features. It is more than a product on its own, but a fully integrated part of the Ordnance Survey data stack:
- Both grid and contours are available for one price;
- Major roads, rail and large water bodies have been modelled with superior accuracy for better results in modelling applications;
- OS Terrain 5 will be updated on a quarterly basis with both COU and full supply options, using the latest integrated content; and
- Revision frequency is planned to take place on a 3-5 year rolling cycle synchronised with our large-scale data.
- Do I have to order grid and contours?
The delivery of OS terrain 5 has been designed to offer both grid and contours for one price to save you having to manage separate product orders and licences. There is a choice of contour formats so you select the type of contours you wish to receive. You will receive links to both grid and contours but are not forced to download any data formats that you do not want. However if you just want grid you do need to choose a contour type to complete the order as the product offers both formats for one price.
- Why is the Isle of Man not included?
The Isle of Man is not captured in our large scale data products, such as OS MasterMap® Topography Layer. OS Terrain products have been created from this same source data to ensure consistency throughout our products. The Isle of Man has a separate mapping agency; please visit the gov.im website for more information.
- Can the data be translated to another format?
There is no reason why the data could not be translated to meet your software requirements. We cannot advise on any specific translators, but you might like to speak with your system supplier, a partner or search the internet for suitable commercial or open source translators. A good place to start is our Partners page.
- Is there any third party content, e.g. lidar?
No, the source data was captured by Ordnance Survey Integrated Capture Programme suppliers and is wholly Ordnance Survey intellectual property.
- Why is the grid available as GML if this cannot be translated?
Ordnance Survey is committed to open data formats and seeks to comply with INSPIRE. GML is a complex language which provides additional functionality in a non-vendor-specific format.
The OS Terrain .gml file effectively provides metadata (such as location, grid spacing and the vertical reference system). It also contains spatial reference information in a software independent form. The data itself is provided as an ‘external data block’, that is, the ASCII file. Currently, common software packages do not support GML in this form but the ASCII grid data can be used alone.
- Why are there fewer contour data files?
There are 10,579 grid files and 10,572 contour data files available in OS Terrain 5. This is due to the lack of real-world changes in height in some very flat areas. There are no contours available to populate data files but we have created metadata files for these tiles to provide relevant information such as update dates etc.
These tiles are as follows:
These tiles appeared in Land-Form PROFILE because this contained additional spot heights captured down features such as roads and because PROFILE was supplied in NTF and DXF formats which allowed both point and line geometries in the same file.
- Where are the gml schema files?
They are contained within the gml file and located here:
- What does pixel-centred mean?
The OS Terrain products differ from the previous Land-Form products because the pixel-centred grid provides one height value for a whole cell, either 5m2 or 50m2. The height for the whole square or pixel has been taken from the height of the source data taken from the centre of the square. In previous products, such as, Land-Form Profile, the heights were represented by the corner values of the tiles and had overlaps on tile edges.
The source data from which we derive the products contains mass points and break lines in a triangulated model called a Triangulated Irregular Network (TIN). To create the grid we evaluate the TIN surface that falls within each grid square. Therefore each z-value in the grid will be the height at that point on the TIN. We employ barycentric interpolation, which is a standard method of interpolating between triangles of a TIN. Because the grid point represents just one height, the value given applies for the whole cell.
- Why do some products have different heights on hill summits?
Our products have been developed from different sources to suit their scale and origin. We are developing a process to include our authoritative source of known summit heights, as would be declared in our small-scales mapping such as 1:50 000 scale into the spot heights for our terrain models and other products. This would be an enhancement to the contour products (the grid is an interpolation of the whole surface presented as a regular grid of values and would not represent a stated summit).
Once developed, we aim to make these heights available for use in products such as Vector Map District and our new height family OS Terrain.
- Why is the sea not represented as one value?
In OS Terrain, the mean high and mean low water lines have been assigned constant height values, based on the average for each tile from information sourced from tide tables. These values have been continued offshore up to the tile edge to ensure consistency (but do not affect the onshore values).
Due to local tidal conditions, the height of the mean high and low water mark varies continuously around the coast of Britain. Inevitably, this means that there can be a step from the tidal area between adjacent tiles.
OS Terrain 5 is regularly updated in line with other large-scale datasets. It explicitly and accurately models significant features such as roads, railways, quarries and lakes, offering a typical accuracy exceeding 2 m RMSE.