OS Terrain 50 support

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:

1) OS Terrain 5™
A mid-resolution DTM product, designed to be interoperable with our large-scale data.

2) OS Terrain 50®
A lower-resolution DTM product, designed for landscape visualisation and analysis over large areas. The grid format is now available through OS OpenData™.

No, OS Terrain products originate from a completely new capture flowline designed to use imagery flown for our large-scale products to produce height data for a new height content store. The store has been designed as the base for our 3D strategy to enable the height data to be used throughout our portfolio and for new products in the future.

No, the source data was captured by Ordnance Survey Integrated Capture Programme suppliers and is wholly Ordnance Survey intellectual property.

OS Terrain 50 is published in both grid and contour formats. Both data types are created from the same source data and are supplied as tiles. Currently only the grid data product is available to download.

Here are some details on the two formats:

• OS Terrain 50 grid - a grid of heighted points with regular 50m post spacing
• OS Terrain 50 contours - a contour dataset of 10m standard contour polygon features, which includes, mean high and low water boundaries and spot heights.

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.

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.

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.

They are contained within the gml file and located here:

Grid products

Contour products

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.

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.

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. If you wish to remove this detailed information and apply other values for the sea, you can select the mean high and low water boundaries from the contour product and use them as a bounding box.