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OS Terrain 5 technical specification
The purpose of the technical specification is to:
Provide a brief description of the presentation of OS Terrain 5 in its supply formats.
Provide licensed system suppliers with as much detail as necessary to enable OS Terrain 5 files to be easily understood and processed by application software.
Product overview
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 Open Data product and, as such, it is free to view, download and use for commercial, educational, and personal purposes.
Available formats
OS Terrain 5 is published as both grid data and contour data in a variety of formats. Both data types are created from the same source data and are supplied as 5km-by-5km tiles. These tiles are identified by quoting the National Grid reference of the south-west corner of the area they cover.
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. This includes mean high water and mean low water boundaries and spot heights.
OS Terrain 5 is available to download in the following formats:
5m grid in ASCII grid and GML 3.2.1 (simple features profile – level 0).
5m contours and spot heights in GML 3.2.1 (simple features profile – level 0).
5m contours and spot heights in Esri shapefile.
Unique identifiers
GML 3.2 requires features and their geometries to have unique identifiers. For OS Terrain products, the feature identifiers have been structured as follows: os.t5.<tile name>.<sequential number>, where the second part abbreviates the product name. Geometry identifiers in the GML use the same form, but with a .geom suffix.
Therefore, for a given release of the product, every feature and geometry is guaranteed to have a unique identifier. The products will be updated by whole tile refresh and there are no plans to supply feature-based change only updates. When a tile is updated, the sequential identifiers are regenerated.
Coordinate reference system
The coordinate reference system for geometries in the OS Terrain GML is expressed using an EPSG (European Petroleum Survey Group) code embedded in a uniform resource name (URN) (urn:ogc:def:crs:EPSG::27700). This is a more generic way of expressing the reference system, rather than osgb:BNG (British National Grid), used in previous Ordnance Survey products.
Index
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