OS Terrain 50

User guide
Contents
Section Page no
Preface .................................................................................................................................................. 3
Contact details ........................................................................................................................... 3
Use of the product ...................................................................................................................... 3
Purpose and disclaimer............................................................................................................... 3
Copyright in this guide ................................................................................................................ 4
Data copyright and other intellectual property rights ................................................................... 4
Trademarks ................................................................................................................................ 4
Back-up provision of the product ................................................................................................. 4
Using this guide .......................................................................................................................... 4
Chapter 1 Introduction .............................................................................................................................. 5
Using this guide ..................................................................................................................... 5
OS Terrain overview .................................................................................................................... 5
OS Terrain 50 ......................................................................................................................... 5
Applications .......................................................................................................................... 5
Features ................................................................................................................................ 5
Accuracy ............................................................................................................................... 6
Supply format........................................................................................................................ 6
Ordering data ........................................................................................................................ 6
Coverage and file sizes ........................................................................................................... 6
File structure ......................................................................................................................... 7
INSPIRE compliance .............................................................................................................. 8
Computer hardware............................................................................................................... 8
Computer software ................................................................................................................ 8
Chapter 2 OS Terrain 50 ............................................................................................................................. 9
Grid ............................................................................................................................................ 9
Introduction .......................................................................................................................... 9
Contours ..................................................................................................................................10
Introduction ........................................................................................................................10
Feature types.......................................................................................................................10
Spot heights ........................................................................................................................11
Chapter 3 Data, metadata, and additional files .......................................................................................12
Metadata ..................................................................................................................................13
Metadata Viewing stylesheet ................................................................................................14
Chapter 4 OS Terrain 50 source data .......................................................................................................15
Introduction .............................................................................................................................15
Coverage .............................................................................................................................15
Positional accuracy requirements ........................................................................................15
Modelling of features in source data ..........................................................................................16
Chapter 5 The National Grid ....................................................................................................................18
Annexe A Product and service performance report form .......................................................................19

v1.4 – 04/2019

D05300_49

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Preface
This user guide (hereafter referred to as the guide) is designed to provide an overview of OS Terrain 50 (hereafter
referred to as the product) and it gives guidelines and advice on how a customer might derive the maximum benefit
from the product. It assumes a general knowledge of geographic information. If you find an error or omission in this
guide, or otherwise wish to make a comment or suggestion as to how we can improve the guide, please contact us
at the address shown below under contact details or complete the product and service performance report form at
annexe A and return it to us.

Contact details
Our Customer Service Centre will be pleased to deal with your enquiries:
Customer Service Centre
Ordnance Survey
Adanac Drive
SOUTHAMPTON
SO16 0AS
General enquiries (calls charged at local rate): +44 (0)3456 050505
Dedicated Welsh Language HelpLine: 03456 050504
Textphone (deaf and hard of hearing users only please): +44 (0)2380 056146
Email: customerservices@os.uk
or visit the Ordnance Survey website at: www.os.uk
This document has been screened in accordance with the requirements set out in Ordnance Survey's Equality
Scheme. If you have difficulty reading this information in its current format and would like to find out how to access
it in a different format (Braille, large print, computer disk or in another language), please contact us on:
+44 (0)3456 050505.

Use of the product

The product is made available to you and your organisation on the terms of the Open Government Licence at
www.os.uk/opendata/licence.

Purpose and disclaimer

This guide is provided for guidance only and does not constitute any warranty, representation, undertaking,
commitment or obligation (express or implied) about the product or its suitability for any particular or intended
purpose. Any warranties, representations, undertakings, commitments and obligations given by Ordnance Survey
about the product and/or its suitability for any particular or intended purpose are set out in your customer
contract. It is your responsibility to ensure that this product is suitable for your intended purpose.
Ordnance Survey does not accept any liability (whether for breach of contract, negligence or otherwise) for any loss
or liability you or any third party may suffer in relying on this guide and any guidance, suggestion, advice or
explanation provided in it. Any liability that Ordnance Survey has to you in relation to the product, its supply, use,
accuracy, data supplied, functionality or any other liability arising out of or in connection with the product is
limited as set out in your customer contract.
We may change the information in this guide at any time without notice.
We do not accept responsibility for the content of any third-party websites referenced or accessed in or through this
guide, any contractual documentation, and/or the Ordnance Survey website.

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Copyright in this guide
This guide (including for the avoidance of doubt any mapping images reproduced herein), is © Crown copyright
2013. All rights reserved.
Any part of this guide may be copied for use internally in your organisation or business so that you can use the
product for the purpose for which it is licensed to your organisation or business (but not otherwise).
No part of this guide may be reproduced or transmitted in any form or by any means (including electronically) for
commercial exploitation without the prior written consent of Ordnance Survey.
No part of this guide may be copied or incorporated in products, services or publications that you generate for
onward sale, or as free promotional or support materials, without the prior written consent of Ordnance Survey.

Data copyright and other intellectual property rights

The Crown (or, where applicable, Ordnance Survey’s suppliers) owns the intellectual property rights in Ordnance
Survey digital map data.
Full details of the terms and conditions under which Ordnance Survey digital map data may be processed and/or
manipulated or copied by a customer – whether or not for use on PCs or workstations or for making hard copies –
are available from the Customer Service Centre. You should check the terms and conditions with us before using
the data. It is also the responsibility of the holder of the digital map data to ensure that any plotted or printed
output contains the required copyright and database acknowledgements in a conspicuous position.

Trademarks
Ordnance Survey, the OS Symbol, OS MasterMap, OS MasterMap Integrated Transport Layer, OS OpenData, Land-
Form PANORAMA and Land-Form PROFILE are registered trademarks and OS Terrain 5 and OS Terrain 50 are
trademarks of Ordnance Survey, the national mapping agency of Great Britain.
Esri is a trademark of Esri Inc.

Back-up provision of the product

You are advised to copy the supplied data to a back-up medium.

Using this guide

The documentation is supplied in portable document format (PDF) only. Free Adobe® Acrobat Reader® software,
which displays the guide, incorporates search and zoom facilities and allows you to navigate within. Hyperlinks are
used to navigate between associated parts of the guide and to relevant Internet resources by clicking on the blue
hyperlinks and the table of contents.
If you are unfamiliar with any words or terms used and require clarification, please refer to the glossary at the end
of the document.

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Chapter 1 Introduction
Using this guide
This document consists of:
• a user guide which contains basic information you will need to understand, use and manage OS Terrain 50; and
• a technical specification which contains detailed technical information about the data formats.
Annexe A in the user guide contains a product and service performance report form for you to submit any
comments on OS Terrain 50. You may also email us at opendata@os.uk

OS Terrain overview
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:
• OS Terrain 5 a mid-resolution DTM, designed to be interoperable with our large-scale data; and
• OS Terrain 50 a lower-resolution DTM product, designed for landscape visualisation and analysis over large
areas. This is available through OS OpenData™.

OS Terrain 50
OS Terrain 50 is published as both grid and contours each in a variety of formats. Both data types are created from
the same source data and are supplied as 10 km by 10 km tiles.
• OS Terrain 50 grid a grid of heighted points with regular 50 metre post spacing.
• OS Terrain 50 contours a contour dataset of 10 metre interval standard contour polylines which includes,
mean high and low water boundaries and spot heights.

Applications
OS Terrain 50 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 and it can be used in conjunction with many other Ordnance Survey digital products,
for analytical, modelling, planning and visual purposes to enable:
• View-shed modelling.
• Asset/site management.
• Visual aid.
• Signal propagation.
• Environmental analysis.
• High level development.
• Geological analysis.
• Line of sight planning.
• Fly-through sequences.

Features
• Maintained within our integrated 3–5 year flying programme and synchronised with our other product updates.
• Specific modelling of significant features, particularly networks.
• Available in a variety of formats.
• Full coverage of Great Britain.
• Provided free at the point of use as a download through OS OpenData™.

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• Designed to work with Ordnance Survey’s small-scale data.
• Supplied as both grid and contours with spot heights.
• Annual product updates.
• The foundation for Ordnance Survey’s three-dimensional strategy.
• Available in GML 3.2, using Open Geospatial Consortium (OGC) Simple Features Profile (level 0).

Accuracy
OS Terrain 50 has been compared with GPS points in a range of sample areas to provide a Route Mean Square Error
(RMSE) value for the height points in each geographic area; urban and major communication routes, rural and
mountain and moorland. OS Terrain 50 grid has been verified to be 4 m RMSE.

Supply format
OS Terrain 50 is available as:
• GeoPackage (OS Terrain 50 contours only)
• 50 metre grid in ASCII grid and GML 3.2.1 (Simple Features Profile – level 0).
• 10 metre contours in Esri® shapefile or GML 3.2.1 (Simple Features Profile – level 0).

Ordering data
The data is provided as a full set of Great Britain in tiles and is available as download only. The national dataset is
supplied as 10 km by 10 km tiles of data. There are 2,858 tiles in the product arranged into 55 folders representing
each 100 km tile grid square.
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.
This data is designed to be kept up to date by annual full tile resupply. Please visit the OpenData pages to view the
update dates.

Coverage and file sizes

OS Terrain 50 is full national coverage of Great Britain. Terrain 50 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. Compression rates vary for
contour tiles, dependant on the number of features in the geographic location.
A full national supply of OS Terrain grid is 153 Mb compressed.

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File structure
When ordered from OS OpenData, the files will be arranged within folders. The following example is for OS Terrain
50 in grid format:

terr50_gagg_gb.zip

Licence.txt
data

[100 km tile ref]

[product Name and Format]

[10 km tile ref] [publication
date].zip

Metadata_
[TILE REF].xml

http://www.ordnancesurvey.co.uk/oswebsite/

[TILE REF]
.asc aux.xml

OSTerrain_Metadata_
viewing_stylesheet.xsl
[TILE REF].gml

OSTerrainGrid
[TILE REF].prj Products.xsd

[TILE REF].asc

In the diagram above some files are located on the Ordnance Survey website. Please see Data, metadata and
additional files for more information.

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INSPIRE compliance

Terrain 2 conforms to the INSPIRE Elevation specification which itself is based on common open standards with the
exception that it is only available in British National grid.

Computer hardware
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 CAD to mainframe computers with specialised
translators and applications. Please see Coverage and file sizes for more information. Your system supplier will be
able to advise on your requirements.

Computer software
OS Terrain 50 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.

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Chapter 2 OS Terrain 50
This chapter describes the structure of the data in each format.

Grid
Introduction
The height data is presented as a raster dataset of height values which are calculated at the centre of the pixel. This
method of creating the data means there are no overlaps between tiles nor common values along the edge.
Coordinate reference systems for DTMs may be used to calculate the DTM origin and coordinates of individual
posts.

OS Terrain 50 posts displayed as a heighted, shaded raster in a GIS.

Mean high and low water representation in grid

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, in the grid, 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.

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Contours
Introduction
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.

OS Terrain 50 contours and spot heights displayed as a heighted raster in a GIS.

Feature types
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
withdrawn Ordnance Survey’s product, Land-Form PROFILE.

Mean high and low water representation in contours

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 ‘meanHighWaterSprings’ and ‘meanLowWaterSprings’ for
Scotland. This attribution allows their display parameters to be changed to show individual features, as desired.

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Spot heights
These have been created using an algorithm that selects the highest source data point within every enclosed
contour. As they are from an interpolated surface of the real world they cannot be guaranteed as summits or
highest points of the feature but it is intended that, in time, the height source data will be enriched by additional
spot height data. 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 as 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.

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Chapter 3 Data, metadata, and additional files
The product will be supplied separately for grid or contour as compressed folders for each geographic tile of data.
Each compressed folder will consist of data plus a number of additional files. This chapter describes these files and
their purpose.

Grid data (ASCII and GML)

<tile name>.asc Esri ASCII grid data.
<tile name>.gml OGC file for the ASCII grid to enable the data to be loaded as
GML format.
<tile name>.prj File containing the spatial reference system in a format
defined by Esri.
<tile name>.asc.aux.xml A file which provides parameters to enable default styling in
Esri applications. This ensures that the shading is
consistent across the data but allows you to apply your own
choice of colour ramp.
Metadata_<tile name>.xml A metadata file for grid data, providing information on the
flying date and so on.

Contour data (GML)

<tile name>.gml GML data file
Metadata_<tile name>.xml A metadata file for contour data providing information on
the flying date and so on.

Contour data (Esri shapefile)

<tile name>_line.shp Esri shapefiles for contour and tide line data.
<tile name>_line.dbf
<tile name>_line.shx
<tile name>_line.prj
<tile name>_point.shp Esri shapefiles for spot height data.
<tile name>_point.dbf
<tile name>_point.shx
<tile name>_point.prj
Metadata_<tile name>.xml A metadata file for contour data providing information on
the flying date and so on.

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Metadata
The XML metadata for the new terrain products follows the GEMINI metadata standard, which Ordnance Survey has
committed to for the UK Location Programme and INSPIRE. Ordnance Survey has provided metadata for national
sets of products (https://www.ordnancesurvey.co.uk/xml/products/). OS Terrain 5 and OS Terrain 50 will also have
metadata in this form, on a per-tile basis.

The XML metadata contains comments to clarify the meaning of the XML tags used in the file.

Name Description Examples

Product identification
Product name OS Terrain 50 gmd:MD_DataIdentification/gmd:citation/gmd:CI_
Citation/gmd:collectiveTitle/gco:CharacterStri
ng
Spatial The structure of the data, either gmd:MD_DataIdentification/gmd:spatialRepresent
ationType/gmd:MD_SpatialRepresentationTypeCode
representation grid (DTM) or vector (contours)
Tile reference 10 km National Grid tile reference gmd:MD_DataIdentification/gmd:citation/gmd:CI_
Citation/gmd:title/gco:CharacterString
Topic category INSPIRE theme: elevation gmd:MD_DataIdentification/gmd:topicCategory/gm
d:MD_TopicCategoryCode
Coordinate The projected Coordinate gmd:MD_ReferenceSystem/gmd:referenceSystemIden
tifier/gmd:RS_Identifier/gmd:code/gmx:Anchor
reference systems Reference System British National xlink:href="urn:ogc:def:crs:EPSG::27700"/Briti
Grid and the Vertical Reference sh National Grid
System Ordnance Datum Newlyn gmd:MD_ReferenceSystem/gmd:referenceSystemIden
tifier/gmd:RS_Identifier/gmd:code/gmx:Anchor
xlink:href="urn:ogc:def:crs:EPSG::5701"/Ordnan
ce Datum Newlyn
Change history
Flying date(s) The date that the area was flown gmd:MD_DataIdentification/gmd:extent/gmd:EX_Ex
tent/gmd:temporalElement/gmd:EX_TemporalExtent
by Ordnance Survey for revision. /gmd:extent/gml:TimePeriod
To accommodate multiple flying /gml:beginPosition
dates within the tile, two values /gml:endPosition
will be recorded, earliest flying
date then latest flying date
present. Both dates can be
identical.
For Profile content the Date Flown
will be recorded as <null>
Processing date The date the tile was created by gmd:MD_DataIdentification/gmd:citation/gmd:CI_
Citation/gmd:date/gmd:CI_Date/gmd:date/gco:Dat
Ordnance Survey, not the date of e
the real-world change or survey
Version number An incrementing number to gmd:MD_DataIdentification/gmd:citation/gmd:CI_
Citation/gmd:edition/gco:CharacterString
indicate the number of times the
tile has been published
Reason for change This provides information about gmd:MD_DataIdentification/gmd:citation/gmd:CI_
Citation/gmd:date/gmd:CI_Date/gmd:dateType/
the update of the data and gmd:CI_DateTypeCode
whether it is creation (new) or a
revision (modified/verified) which
is described in the metadata by
using lineage below
Lineage Text to describe the current status gmd:DQ_DataQuality/gmd:lineage/gmd:LI_Lineage/
gmd:statement/gco:CharacterString
of the tile, either: ‘created from
new imagery’, ‘some parts revised
from new imagery’, ‘new imagery
examined and no change’

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Metadata Viewing stylesheet

To make the xml easier to read an XSLT viewing style sheet is provided (OSTerrainMetadataViewingStylesheet.xsl),
which converts the XML to HTML for ease of viewing in a web browser. Some browsers and other software will read
this automatically if the user is connected to the internet as its address is referenced in the metadata but it can also
be located on the Ordnance Survey website
http://www.ordnancesurvey.co.uk/xml/stylesheet/OSTerrainMetadataViewingStylesheet.xsl.

Metadata xml file example

This is a section of the xml file in its native format with the location of the style sheet highlighted. It can be read like
this when opened in an xml viewer or basic file reader.

<?xml version="1.0" encoding="UTF-8"?>

<?xml-stylesheet type="text/xsl"
href="http://www.ordnancesurvey.co.uk/xml/stylesheet/OSTerrainMetadataViewingStylesheet.xsl"?>

<gmd:MD_Metadata xmlns:gmd="http://www.isotc211.org/2005/gmd"
xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:gmx="http://www.isotc211.org/2005/gmx" xmlns:gco="http://www.isotc211.org/2005/gco"
xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="http://www.isotc211.org/2005/gmx
http://eden.ign.fr/xsd/isotc211/isofull/20090316/gmx/gmx.xsd"><!-- Unique identifier, required
if this record is being used in a metadata management system -->
<gmd:fileIdentifier>
<gco:CharacterString>OSTerrain5.NT23NE</gco:CharacterString>
</gmd:fileIdentifier>
<gmd:contact gco:nilReason="missing"></gmd:contact>
<gmd:dateStamp>
<gco:DateTime>2013-01-08T03:22:25</gco:DateTime>
</gmd:dateStamp><!-- Projected Coordinate Reference System -->
<gmd:referenceSystemInfo>
<gmd:MD_ReferenceSystem>
<gmd:referenceSystemIdentifier>
<gmd:RS_Identifier>
<gmd:code>
<gmx:Anchor
xlink:href="urn:ogc:def:crs:EPSG::27700">British National Grid</gmx:Anchor>

Metadata xml file referencing the XSLT viewing stylesheet example

This is the same section of the metadata file above when viewed directly by clicking on the file, this utilising the
style sheet.

OS Terrain Tile Metadata

Product identification:
Product name: OS Terrain 5
Spatial representation: grid
Tile reference: NT23NE
Topic category: elevation
Coordinate reference systems: British National Grid

Esri grid styling – asc.aux.xml file

This XML file contains min, max, mean and standard deviation height values for the product, in a format defined by
Esri. The same values are supplied for every tile. Providing these height statistics is intended to allow colour ramps
to be applied by the user, such that adjacent tiles are styled consistently.

This can be disabled by removing the asc.aux.xml file from each downloaded folder or adjusting the parameters of
the minimum and maximum heights in a GIS if desired.

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Chapter 4 OS Terrain 50 source data

Introduction
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 which we endeavour to achieve in
the source data.
The grid and contour products are both interpolated from this source TIN model. This is because the TIN model is
not widely supported by GIS software. As OS Terrain 50 is designed to work with small-scale products the feature
modelling will have a more generalised representation in the product.

Coverage
The minimum coverage of the data extends out to the low water mark, defined by Hydrographic Office tables with a
height value for each 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.

Positional accuracy requirements

The z values of the source TIN data have to 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 Newlyn Datum must achieve the root mean square error (RMSE) set for each area.

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Modelling of features in source data
Representation of the surface
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. The vast majority of areas will record a height relative to Ordnance Survey Newlyn Datum. 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 (those expected to remain until the next revision period or longer) are modelled.

Underground and overhead features

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.

Terrain smoothness
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 track bed,
will also appear smooth in the data.

Edgematching
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
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

Vegetation
Areas of vegetation, such as hedgerows and trees are removed to ensure the bare earth surface is correctly
recorded.

Vertical features
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

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.

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Manmade landforms associated with mineral workings and landfill
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

Contained water bodies greater than 0.7ha

In order to respond to the Flood and Water Management Act 2010 the extent of all flat water bodies that are > 0.7ha
in area (that is, greater than 7000 cubic metres capacity) must 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.

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Chapter 5 The National Grid
OS Terrain 50 is supplied as a whole set of Great Britain divided into 10 km by 10 km 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 100 km by 100 km. Each of these squares has a unique two-letter reference, for example,
TG in the diagram below.

To describe a 10 km by 10 km tile,
first add a two digit reference to
the 100 km by 100 km 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’s website
at:
http://www.ordnancesurvey.co.uk

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Annexe A Product and service performance report form
Ordnance Survey welcomes feedback from its customers about OS Terrain 50.
If you would like to share your thoughts with us, please print a copy of this form and when completed post or fax it
to the address below.
Your name: ..........................................................................................................................................................................
Organisation: .......................................................................................................................................................................
Address: ...............................................................................................................................................................................
..............................................................................................................................................................................................
..............................................................................................................................................................................................
Postcode: .............................................................................................................................................................................
Phone: ..................................................................................................................................................................................
Fax: ......................................................................................................................................................................................
Email: ...................................................................................................................................................................................
Quotation or order reference: .............................................................................................................................................
Please record your comments or feedback in the space below. We will acknowledge receipt of your form within
three (3) working days and provide you with a full reply or a status report within 21 working days.

If you are posting this form, please send it to:

OS Terrain 50 Product Manager, Ordnance Survey, Adanac Drive, SOUTHAMPTON, SO16 0AS.
If you wish to return it by fax, please dial 023 8005 6159.
Any personal information that you supply with this report form will be used by Ordnance Survey only in the
improvement of its products and services. It will not be made available to third parties.

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OS Terrain 50
Technical specification
Contents
Section Page no
Introduction 21
Copyright in this specification ...................................................................................................21
Chapter 1 Technical specification introduction .......................................................................................22
Chapter 2 GeoPackage for Terrain 50 contours .......................................................................................23
GeoPackage overview ...............................................................................................................23
GeoPackage Attribute table .................................................................................................24
Chapter 3 ASCII grid for OS Terrain 50 grid ..............................................................................................25
ASCII grid overview ...................................................................................................................25
Chapter 4 GML for OS Terrain 50 grid and contours ................................................................................26
GML overview ...........................................................................................................................26
GML grid for OS Terrain 50 .........................................................................................................27
GML contours and spot heights for OS Terrain 50 .......................................................................27
Detailed GML model .............................................................................................................27
Spot heights ........................................................................................................................27
GML feature types ................................................................................................................27
Chapter 5 Esri shapefile for OS Terrain 50 contours ................................................................................30
Esri shapefile overview ..............................................................................................................30
Detailed shapefile model .....................................................................................................30
Annexe A Glossary ..................................................................................................................................32

v1.4 – 04/20197

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Introduction

Copyright in this specification

This specification, (including for the avoidance of doubt any mapping images reproduced herein), is
© Crown copyright 2013. All rights reserved.
Any part of this specification may be copied for use internally in your organisation or business so that you can use
OS Terrain 50 for the purpose for which it is licensed to your organisation or business (but not otherwise).
No part of this specification may be reproduced or transmitted in any form or by any means (including
electronically) for commercial exploitation without the prior written consent of Ordnance Survey.
No part of this specification may be copied or incorporated in products, services or publications that you generate
for onward sale, or as free promotional or support materials, without the prior written consent of Ordnance Survey.

OS Terrain 50 technical specification introduction v1.4 – 04/2019 © Crown copyright Page 21 of 35

Chapter 1 Technical specification introduction
The purpose of the technical specification is to:
• provide a brief description of the presentation of OS Terrain 50 in its supply formats; and
• provide Licensed System Suppliers with as much detail as necessary to enable OS Terrain 50 files to be easily
understood and processed by application software.

OS Terrain 50 is available to download as:

• 10 m contours in GeoPackage.
• 50 m grid in ASCII grid and GML 3.2.1 (simple features profile – level 0).
• 10 m contours in GML 3.2.1 (simple features profile – level 0).
• 10 m contours in Esri® shapefile.

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Chapter 2 GeoPackage for Terrain 50 contours

GeoPackage overview
Geopackage is an open, standards-based, portable, platform-independent, compact format for transferring
geospatial information. It is designed to be a lightweight format that can contain large amounts of varied and
complex data in a single, easy to distribute and ready to use file.

GeoPackage is a contour dataset of 10 metre interval standard contour polylines which includes, mean high and
low water boundaries and spot heights.
It can hold large amounts of data and can support the different geometry types and indexes which make the data
much easier to use.
GeoPackage offer users the following benefits:

• The single file is easy to transfer and offers the end-user a rich experience.
• No file size limit so lots of data can be easily accommodated.
• Supports raster, vector and database formats making it a highly versatile solution.
• It is an Open Geospatial Consortium (OGC) Standard.
• In most cases, it is a plug-in-and-play.

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GeoPackage was released in 2014 and so is a relatively new format. As a result, some older software packages may
have trouble loading it or may need a plugin in order to do so. If this is the case, your version of GIS may need
updating. For example, QGIS software, as of version 2.18 (October 2016), can interact with GeoPackage files without
needing additional plugins or settings. Earlier versions will either require a plugin or will not be able to interact with
this format.

GeoPackage Attribute table

Example of attribute headers.
fid id propertyValue PropertyValue_uom spotHeightType
1 Os.t50.hp40.0 14 m generic
2 Os.t50.hp40.1 28 m generic

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Chapter 3 ASCII grid for OS Terrain 50 grid

ASCII grid overview

ASCII grid is a generic, text-based DTM format which was originally developed by Esri and it is sometimes referred to
as ArcInfo ASCII grid or ArcGrid ASCII. This data can be read by most standard GIS software without additional
translation.
The term data structure refers to the organisation and sequence of the records in the data file and not to the
geographical topology of the data.

Data structure
Header section
The data is specified as a raster grid with the height values being calculated at the centre of the pixel. To represent
this in ASCII grid format the initial coordinates in the map header originate on the north-west corner of the tile. The
data is presented in rows reading from west to east creating a row of 200 values. The next row will begin 25 m from
the western edge (pixel-centre) 50 m south of the origin and again progressing at 50 m intervals to the east.

Number of points
ncols 200
nrows 200
xllcorner 290000 (example)
yllcorner 80000 (example)
cellsize 50

Data section
The height values are presented in the standard ASCII grid format as a series of real values. The height values are
given to the nearest 0.1 metre.

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Chapter 4 GML for OS Terrain 50 grid and contours
This chapter describes how OS Terrain 50 is defined in GML version 3.2.1. An understanding of XML and XML schema
is required.

GML overview
The OpenGIS® GML Encoding Standard, the GML is an XML grammar for expressing geographical features. GML
serves as a modelling language for geographic systems as well as an open interchange format for geographic
transactions on the Internet. As with most XML based grammars, there are two parts to the grammar – the schema
that describes the document and the instance document that contains the actual data. A GML document is
described using a GML Schema. This allows users and developers to describe generic geographic data sets that
contain points, lines and polygons.
http://www.opengeospatial.org/standards/gml/

The GML conforms to GML 3.2.1 Simple Features (level 0) which is a subset of the full GML specification, intended to
make it easier for GIS vendors to provide a minimum level of support for GML.

GML schema
XML schemas are used to validate the format and content of the GML. The GML specification provides a set of
schemas that define the GML feature constructs and geometric types. These are designed to be used as a basis for
building application-specific schemas, which define the data content.
The OS Terrain 50 application schemas, which are referenced by the data, are available at
http://www.ordnancesurvey.co.uk/xml/terrainschema/
The user may need to be connected to the internet to access these online schemas while working with OS Terrain
50 in GML unless their software supports local copies of the schema. Depending upon the software that is being
used to read the data the user has the following options:
• The software does not use the schema, therefore does not need to be connected to the Internet.
• The software needs the schema, but can reference the schema from a local copy, if it is downloaded in advance.
• The software needs the schema and can only reference it from the online version, therefore needs to be
connected to the Internet.

Feature types

The model is ‘feature based’ so that ‘ContourLine’, ‘SpotHeight’ and ‘LandWaterBoundary’ are feature types with
specific attributes. Please see GML contours and spot heights for more information.

Coordinate reference system

The coordinate reference system for geometries in the OS Terrain GML, is expressed using an EPSG code embedded
in a 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.

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.t50.<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-
update. When a tile is updated, the sequential identifiers are re-generated.

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GML grid for OS Terrain 50
Ordnance Survey is committed to open data formats. The grid data is supplied as ASCII with GML, to enable their
use in either format. The GML file does not contain any spatial height data as this data has been provided as an
‘external data block’, that is, the ASCII grid file. The .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.
Currently, common software packages do not support it in this form but the ASCII grid data can be used alone.

GML contours and spot heights for OS Terrain 50

Detailed GML model

Spot heights
These have been created by an automated process to select the highest mass point within every closed contour. If
the point is the same height as the contour it will be removed. It is possible that some spot heights could be lower
than the surrounding contour due to genuine depressions. The z value is rounded to 0 decimal places.

GML feature types

The feature types within the contours enable their representation as individual features to enable easier
interpretation, for example, the contours can be drawn as different colours to highlight the index contours, the high
water mark and spot heights over a certain value.
The column names have been reformatted to facilitate compliance with Esri software.

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Column name Type Description
ContourLine
+geometry GM_LineString The structure of the feature
+propertyValue Distance The length of the contour
+contourLineType String master
ordinary
auxiliary
Notes The ContourLine sub type value names align with INSPIRE draft elevation
specification. The terms master and ordinary represent the more
traditionally recognised terms index and standard (contours) respectively.
SpotHeight
+geometry GM_Point The structure of the feature
+propertyValue Distance The z value (height above Newlyn, or
other British height datum) of the
feature
+spotHeightType String formSpot
generic
mountainPass
summit
Notes At launch only the sub type value generic has been used but the other values
provide functionality to enrich the attribution if required in a later product
release.
LandWaterBoundary
+geometry GM_LineString
+propertyValue Distance
+waterLevelCategory String meanHighWater
meanLowWater
meanHighWaterSprings
meanLowWaterSprings
Notes Mean high and low waters apply to tidal waters in England and Wales and
the Mean high and low water springs apply to those in Scotland.

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Example of the ContourLine feature types
<os:member>
<os:ContourLine gml:id="os.t50.sx98.691">
<os:geometry>
<gml:LineString srsName="urn:ogc:def:crs:EPSG::27700"
gml:id="os.t50.sx98.691.geom">
<gml:posList>300000 87424.1 299997 87432.8 299995.9 87445.7
299997.1 87459.6 300000 87473.8</gml:posList>
</gml:LineString>
</os:geometry>
<os:propertyValue uom="m">40</os:propertyValue>
<os:contourLineType>ordinary</os:contourLineType>
</os:ContourLine>
</os:member>

Example of the SpotHeight feature types

<os:member>
<os:SpotHeight gml:id="os.t50.sx98.5">
<os:geometry>
<gml:Point srsName="urn:ogc:def:crs:EPSG::27700"
gml:id="os.t50.sx98.5.geom">
<gml:pos>298939.2 88163.4</gml:pos>
</gml:Point>
</os:geometry>
<os:propertyValue uom="m">53</os:propertyValue>
<os:spotHeightType>generic</os:spotHeightType>
</os:SpotHeight>
</os:member>

Example of the LandWaterBoundary feature types

<os:member>
<os:LandWaterBoundary gml:id="os.t50.sx98.138">
<os:geometry>
<gml:LineString srsName="urn:ogc:def:crs:EPSG::27700"
gml:id="os.t50.sx98.138.geom">
<gml:posList>298703.2 80000 298695.7 80002.8 298681.6 80002.1
298662.7 80000</gml:posList>
</gml:LineString>
</os:geometry>
<os:propertyValue uom="m">-0.51</os:propertyValue>
<os:waterLevelCategory>meanLowWater</os:waterLevelCategory>
</os:LandWaterBoundary>
</os:member>

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Chapter 5 Esri shapefile for OS Terrain 50 contours

Esri shapefile overview

The Esri shapefiles consist of eight separate files for the point and line features.

Detailed shapefile model

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File name Description Examples
Contour line files
<tile name>_line.shp Shape format
The contour
geometry
<tile name>_line.dbf Shape attribute ID FEAT_TYPE SUB_TYPE PROP_VALUE
format
os.t50.sy09.113 ContourLine ordinary 80.00
A database file that
contains the os.t50.sy09.114 ContourLine ordinary 100.00
attributes of each
feature os.t50.sy09.115 ContourLine ordinary 70.00

<tile name>_line.shx Shape index format

A positional index
of the features to
enable searching
<tile name>_line.prj Projection format PROJCS["British_National_Grid",GEOGCS["GCS_OSGB_1936",DATUM…

Coordinate
reference system
information
Spot height files
<tile Shape format
name>_point.shp
The spot height
geometry
<tile Shape attribute
name>_point.dbf ID FEAT_TYPE SUB_TYPE PROP_VALUE
format
A database file that os.t50.sy09.0 SpotHeight generic 30.00
contains the
attributes of each os.t50.sy09.1 SpotHeight generic 66.00
feature
os.t50.sy09.2 SpotHeight generic 41.00

<tile Shape index format

name>_point.shx
A positional index
of the features to
enable searching
<tile Projection format PROJCS["British_National_Grid",GEOGCS["GCS_OSGB_1936",DATUM…
name>_point.prj
Coordinate
reference system
information

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Annexe A Glossary
For detailed general information about mapping please visit our Support pages online.

absolute accuracy
A measure that indicates how closely the coordinates of a point in Ordnance Survey map data agree with the true
National Grid coordinates of the same point on the ground. As the true position can never be known exactly, the
statistic is quoted relative to the best known position determined by precise survey methods.

accuracy
The closeness of the results of observations, computations or estimates to the true values or the values accepted as
being true. Accuracy relates to the exactness of the result, and is a measure of the exactness of the operation by
which the result is obtained.

ASCII
American Standard Code for Information Interchange – a standard binary coding system used to represent
characters within a computer.

bit
An acronym for binary digit.

breakline
A line indicating discontinuity in a terrain surface, that is, an abrupt change in gradient.

byte
A unit of computer storage of binary data usually comprising 8 bits, equivalent to a character. Hence megabyte
(one million bytes) and gigabyte (one thousand million bytes).

character
A distinctive mark; an inscribed letter; one of a set of writing-symbols.

contour
A line connecting points of equal elevation.

coordinates
Pairs of numbers expressing horizontal distances along orthogonal axes. Alternatively, triplets of numbers
measuring horizontal and vertical distances.

copyright
Copyright is a legal property right which enables the creator of an original work to protect it from unauthorised use.
Through the Copyright, Designs and Patents Act 1988, Crown copyright continues to subsist in all Ordnance Survey
products until the end of the period of 50 years from the end of the year in which they were published and, in the
case of data, from the end of the year in which it was extracted from the Ordnance Survey database. Crown
copyright is vested in The Controller of Her Majesty’s Stationery Office, who has delegated powers to the Director
General, Ordnance Survey for the administration of copyright in publications and data, including the determination
of the rules and terms under which permission for their reproduction is given.

currency
An expression of the up-to-dateness of data.

data
A representation of facts, concepts or instructions in a formalised manner suitable for communication,
interpretation or processing.

data capture
The encoding of data. In the context of digital mapping this includes map digitising, direct recording by electronic
survey instruments, and the encoding of text and attributes by whatever means.

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data format
A specification that defines the order in which data is stored or a description of the way data is held in a file or
record.

data model
An abstraction of the real world which incorporates only those properties thought to be relevant to the application
or applications at hand. A data model would normally define specific groups of entities and their attributes, and the
relationship between these entities. A data model is independent of a computer system and its associated data
structures.

data point
A coordinate pair that defines the position of a point feature, or one of a series of coordinate pairs that defines a
line feature.

data quality
Attributes of a dataset that define its suitability for a particular purpose, for example, completeness, positional
accuracy, currency, logical structure and so on.

data structure
The defined logical arrangement of data as used by a system for data management; a representation of a data
model in computer form.

data type
This defines the structure of a data item. This in turn determines the range of values it can take and the range of
operations that can be applied to it. Integer, real and character string are examples of data type. Some modern
programming languages allow user-defined types.

database
An organised, integrated collection of data stored so as to be capable of use by relevant applications, with the data
being accessed by different logical paths. Theoretically, it is application-independent, but in reality it is rarely so.

dataset
An identifiable collection of related data.

digital
Data that is expressed as numbers (digits) in computer-readable form is said to be digital.

digital terrain model (DTM)

Also referred to as a digital elevation model (DEM) this primarily defines the ground surface. This will normally
exclude ground surface features such as buildings, woodland and so on.

eastings
See rectangular coordinates.

edgematch
The process of ensuring that data along the adjacent edges of blocks of data matches in both positional and
attribute terms.

feature
An item of detail within a digital map that can be represented by either a point, symbol, text or line.

feature identifier
A unique code to identify an individual feature. A specified part of a record containing a unit of data, such as the
date of digitising. The unit of data may be a data element or a data item.

field
A specified part of a data file containing a unit of data, such as the date of flying.

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format
The specified arrangement of data in a file.

geographical information system (GIS)

A system for capturing, storing, checking, integrating, analysing and displaying data that is spatially referenced to
the Earth. This is normally considered to involve a spatially referenced computer database and appropriate
applications software.
Geopackage (*.gpkg)
Geopackage is an open data format as is defined by the Open Geospatial Consortium (OGC). It is designed to be a
lightweight format that contains both spatial (vector) and metadata tables in a single, ready to use file.

Global positioning system (GPS)

The Navstar global positioning system (GPS) is a constellation of orbiting satellites that provides navigation data to
military and civilian users all over the world. The system is operated and controlled by members of the 50th Space
Wing located at Schriever Air Force Base (AFB), Colorado.

GML
GML was developed by the Open GIS Consortium (OGC), a global organisation of developers and users that aims to
maximise the benefit of geographic information. GML is a spatially enabled dialect of XML schema.

grid
The planimetric frame of reference, for example, the National Grid or the consistent alignment of data points.

kilobyte (Kb)
A total of 1,024 bytes; a measure of data storage capacity.

line
A series of connected coordinated points forming a simple feature with homogeneous attribution.

mean high water/springs (MHW or MHWS)

Depiction of the encroachment of land by tidal waters at mean highest levels – spring tides in Scotland.

mean low water/springs (MLW or MLWS)

Depiction of limits of tidal waters at mean lowest ebb – spring tides in Scotland.

megabyte (Mb)
A total of 1,048 576 bytes; a measure of data storage capacity.

National Grid
The metric grid on a Transverse Mercator projection used by Ordnance Survey on all post-war mapping to provide
an unambiguous spatial reference in Great Britain for any place or entity, whatever the map scale.

Northings
See rectangular coordinates.

Open GIS Consortium (OGC)

This is a global organisation of developers and users that aims to maximise the benefit of geographic information.

origin
The zero point in a system of rectangular Cartesian coordinates.

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photogrammetry
The science, art and technology of obtaining reliable measurements and maps from aerial photographs.

point
A zero-dimensional spatial abstraction of an object represented as a coordinate pair.

point feature
A zero-dimensional spatial abstraction of an object with its position defined by a coordinate pair. Points may also
be represented by symbols, which may have attributes such as orientation and size.

positional accuracy
The degree to which the coordinates define a point’s true position in the world, directly related to the spheroid
and/or projection on which the coordinates system is based.

rectangular coordinates
Also known as x-y coordinates and as eastings and northings. These are two-dimensional coordinates that measure
the position of any point relative to an arbitrary origin on a plane surface, for example, a map projection, a
digitising table or a VDU screen.

Route mean square error (RMSE)

This is the square root of the mean of the squares of the errors between observations, such as GPS points.

Shapefile
This is a data format developed by Esri to describe features such as points, lines and polygons to enable spatial
analysis. A shapefile consists of a number of files designed to hold information essential for the transfer of this data
between software products which are capable of reading shapefiles.

spot height
A point on the Earth’s surface for which the height, above a reference datum, is known and which has been fixed by
observation.

transfer format
The format used to transfer data consistently between computer systems. In general usage this can refer not only
to the organisation of data but also to the associated information, such as attribute codes, which are required in
order to successfully complete the transfer.

Triangulated Irregular Network (TIN)

This is a vector data structure that represents a surface in three dimensions. Masspoints (or z-values) are joined up
as irregular triangles to form a surface to provide a more accurate and less dense method of representing a Digital
Terrain Model. This is because the edges of features are preserved and masspoints are only depicted where
necessary to depict changes in slope.

Extensible Markup Language (XML)

This is a markup language written in a textual data format designed to encode documents and data structures for
transfer over the Internet. It was developed by the World Wide Web Consortium (W3C). XML schemas express shared
vocabularies and allow machines to carry out rules made by people. They provide a means for defining the
structure, content and semantics of XML documents.

Extensible Stylesheet Language Transformations (XSLT)

This is a language for transforming XML documents into objects that can be presented in a format that is more
easily read by the user, such as HTML for web pages or plain text.

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