How does Ordnance Survey map the landscape of Great Britain?
To introduce you to OS Net and the work Ordnance Survey does, an introduction to the two systems used to map the landscape is benefical.
These systems are:
- OS Net Active GPS network RINEX data server (an active digital system that uses GPS and the internet to generate coordiates)
- Passive GPS station database (a passive physical system - recognisable as triangulation pillars or 'trig points')
Each of these systems is detailed below, giving a more detailled explanation of how they operate.
OS Net Active GPS network RINEX data server
By downloading data from the OS Active GPS network via the Internet, and processing this with your own GPS data, you can obtain precise ETRS89 coordinates of your GPS survey stations. Accuracies as high as 1 cm (horizontal) are possible anywhere in Great Britain, and 5 cm horizontal accuracy is routine, using dual-frequency GPS survey equipment and observation periods up to 1 hour (depending on local conditions and the distance to OS Active stations). Vertical accuracy is usually 2-3 times worse than horizontal accuracy, depending on several factors, including the software used.
The Ordnance Survey Active GPS network is a subset of the OS Net network and consists of about 50 permanently installed geodetic GPS receivers throughout Great Britain, such that most locations are within 75 km of at least one OS Active station, and major urban areas are served by several Active stations. All OS Active stations record dual-frequency GPS data 24 hours a day at a 30 second epoch rate. Every hour at about HH:50 the previous full hour of RINEX (Receiver INdependent EXchange) format GPS data from the network is added to Active GPS network RINEX data server for immediate access via this web site.
The photo shows a typical choke ring antenna installation at an OS Active GPS station. This one is on the roof of OS Headquarters in Southampton.
The Active GPS network RINEX data server on this web site allows users to choose which Active stations they wish to download data from (usually those closest to the survey site) and to choose a time range of data to coincide with the survey period. The GPS data is transferred in compressed RINEX format, which is compatible with all GPS processing software.
Typically, the Active network would be used to position a small number of primary survey stations with a dual-frequency survey-grade GPS receiver and processing software, and surveying would then proceed from those primary stations using short-distance relative GPS methods (for example, Real-Time Kinematic, or a DGPS basestation) or an optical survey instrument (for example, total station).
Passive GPS station database
Ordnance Survey's network of passive GPS stations throughout Great Britain allows GPS surveyors to precisely position their survey stations in the ETRS89 coordinate system. OS passive GPS stations are geodetic quality ground marks in user-accessible locations. A typical survey site in Great Britain will have several OS passive stations within 20-35 km (distances may be greater in the Scottish Highlands). The main differences between this network and the Active station network are that passive stations must be occupied by the user's own GPS reference receiver during the survey; and that typical distances from a survey site to the nearest passive stations are smaller than for Active stations, making required observation times shorter and allowing the use of single-frequency GPS receivers.
The Passive GPS station database on this web site is a searchable database of all passive stations, including precise geodetic coordinates, station descriptions and photographs, location sketches, access information, and recent maintenance and monitoring details. This database allows GPS surveyors to plan control surveys based on reliable OS control stations. To use the National GPS Network passive stations, two or more survey GPS receivers (preferably dual frequency) and processing software are required. The resulting ETRS89 GPS coordinates can be precisely converted to National Grid coordinates and Newlyn height coordinates using OS precise transformations (see below).
Why are these networks important?
The Ordnance Survey National GPS Network is the infrastructure which realises (gives access to) our national coordinate systems in Great Britain. The Network consists of the OS Net permanent GPS stations (RINEX data from the Active GPS Network subset is available on this web site), the Passive stations and the coordinate transformation. The national coordinate systems are ETRS89, OSGB36® National Grid, and Ordnance Datum Newlyn (ODN). The National GPS Network has taken over this role from the traditional OS control networks of triangulation stations and levelled bench marks. These traditional control networks are no longer maintained by OS and will eventually be phased out.
The national coordinate systems are used in a great variety of applications by many different user groups. ETRS89 is a precise version of the GPS coordinate system WGS84, and is the standard coordinate system for precise GPS positioning throughout Europe. OSGB36 National Grid is the national standard coordinate system for topographic mapping, including all OS mapping, and for geographically referencing many kinds of information in relation to OS mapping. Ordnance Datum Newlyn (ODN) is the national standard coordinate system for measuring height above mean sea level. ODN is the usual definition of mean sea level in mapping and engineering in Great Britain.
The National GPS Network makes the accurate determination of these national standard coordinates much easier and more efficient for land surveyors, compared to traditional (pre-GPS) surveying methods. It is now possible to determine precise ETRS89 coordinates for your GPS control stations with a single GPS receiver, without ever leaving the survey site. These coordinates can be instantly and precisely transformed to OSGB36, National Grid and ODN height coordinates. Therefore, it is feasible for a greater range of mapping, engineering and environmental projects to use precise national coordinates than was previously possible. This means that in the future, many spatial datasets created at various times, by various organisations, for various reasons, will be directly and precisely compatible with each other.
The aim to increase the intercompatibility and usefulness of spatial datasets throughout Great Britain is what we call 'joined-up geography', and it is a central strategy of Ordnance Survey. The National GPS Network is the most fundamental part of Ordnance Survey's joined-up geography strategy.
How are coordinates transformed using the National GPS Network?
The National GPS Network online coordinate transformer instantly transforms ETRS89 GPS coordinates to OSGB36 National Grid eastings and northings, and orthometric height (MSL) which for mainland Britain is Ordnance Datum Newlyn (ODN). The coordinate transformer uses the National Grid Transformation OSTN02™ and the National Geoid Model OSGM02™. These transformations use more than 2.6 million parameters to precisely model the relationship between ETRS89 and OSGB36/ODN throughout Great Britain.
OSTN02 and OSGM02 are our national standard transformations for converting GPS coordinates to OSGB36 National Grid coordinates and orthometric (MSL) heights, and vice-versa. Together with Active and Passive GPS stations the OSTN02/OSGM02 transformation standards make it easy for GPS surveyors to determine precise coordinates in OSGB36 National Grid coordinates and MSL height as used in Ordnance Survey mapping, without having to occupy traditional control stations (triangulation stations or levelled bench marks) with GPS.
For more information
If you are unsure how to convert National Grid references into full eastings and northings, see our guide to the National Grid.
The OSTN02 and OSGM02 transformation models are also freely available to software developers for incorporation in your own software. An increasing number of popular software packages will include these transformations, which can be recognised by the OS logo with the text 'OSTN02/OSGM02 enabled'.