How does Ordnance Survey map the landscape of Great Britain?
Our surveyors use the OS Net network in to accuratley survey map features using the latest GNSS technology. OS Net based positioning services are available to other surveyors via our commercial partners and recorded OS Net data, in the open standard RINEX format, are freely available to all users from the OS web site.
By downloading data from the OS Net GNSS network via the Internet, and processing this with your own GNSS data, you can obtain precise ETRS89 coordinates of your GNSS 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 GNSS 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 OS Net network and consists of about 115 permanently installed geodetic GNSS receivers throughout Great Britain, such that most locations are within 75 km of at least one OS Net station, and major urban areas are served by several stations. All OS Net stations record dual-frequency GNSS 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 GNSS data from the network is added to OS Net GNSS network RINEX data server for immediate access via this web site.
Typically, the OS Net network would be used to position a small number of primary survey stations with a dual-frequency survey-grade GNSS receiver and processing software, and surveying would then proceed from those primary stations using short-distance relative GNSS methods (for example, Real-Time Kinematic, or a DGPS basestation) or an optical survey instrument (for example, total station).
Why OS Net is important?
OS Net is the infrastructure which realises (gives access to) our national coordinate systems in Great Britain. The national coordinate systems are ETRS89, OSGB36® National Grid, and Ordnance Datum Newlyn (ODN). OS Net taken over the role of realising the national coordinate system 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 These traditional control networks are no longer maintained by OS and will eventually be phased out.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 traditional systems of OSGB36 and ODN are now accessed via OS Net derived ETRS89 and the OSTN15 transformation and OSGM15 geoid model.
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.
OS Net makes the accurate determination of these national standard coordinates much easier and more efficient for land surveyors, compared to traditional (pre-GNSS) surveying methods. It is now possible to determine precise ETRS89 coordinates for your GNSS control stations with a single GNSS 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 online coordinate transformer instantly transforms ETRS89 GNSS 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 OSTN15™ and the National Geoid Model OSGM15™. These transformations use more than 2.6 million parameters to precisely model the relationship between ETRS89 and OSGB36/ODN throughout Great Britain.
OSTN15 and OSGM15 are our national standard transformations for converting GNSS coordinates to OSGB36 National Grid coordinates and orthometric (MSL) heights, and vice-versa. Together with OS Net data the OSTN15/OSGM15 transformation standards make it easy for GNSS 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 GNSS.
The OSTN15 and OSGM15 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. A stand alone software utility for OSTN15/OSGM15 is also available here.
If you are unsure how to convert National Grid references into full eastings and northings, see our guide to the National Grid.