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OS Net and GNSS

What is the difference between GPS and GNSS?

GPS is the United States’ Global Positioning System, arguably the most familiar and widely adopted satellite navigation system. However, it is not the only satellite navigation system available to users.

The term ‘Global Navigation Satellite System’ (GNSS) encompasses not only GPS but also GLONASS (Russia) and the developing BeiDou(China) and Galileo (Europe) satellite navigation systems.

What are the benefits to the user of using GNSS?

GNSS based data collection is much faster than conventional surveying and mapping techniques. Making use of data from networks such as OS Net means a local base station is not necessary, reducing the equipment and staff. Using GNSS can enable the user to:

  • capture position and timing information relative to national and international reference systems;
  • gain efficiencies and cost reductions in time and resource; and
  • have confidence in the results.
What are the applications for GNSS?

Although originally designed as navigation systems, GNSS is now used in a variety of applications.

Commercially they are used as a navigation and positioning tool for machine control, surveying, construction, structural monitoring, aerial surveying and as a timing tool for mobile phones, banking transactions and even controlling electricity grids.

Leisure applications include outdoor recreational activities such as walking, cycling and kayaking.

In the scientific community, GNSS plays an important role in the earth sciences. For example, meteorologists use it for weather forecasting and global climate studies. Geologists can use it as a highly accurate method of surveying and also to measure tectonic motions during and in between earthquakes.

How accurate is GNSS?

Using a single receiver, without any additional corrections, a civilian user can achieve a positional accuracy equal to 5 m–10 m, 95% of the time, and a height accuracy of 15 m–20 m 95% of the time.

Combined with data or corrections from a service such as OS Net, a positional accuracy of 1–2 cms is achievable. (This figure is dependant upon hardware and environmental factors.)

How can I display data captured using GNSS on Ordnance Survey mapping?

GNSS and Ordnance Survey maps use different Earth models and coordinate systems to represent positions. For example, GPS uses the coordinate system WGS84 for position and height, whereas Ordnance Survey maps use the National Grid, which uses the coordinate system OSGB36 and Ordnance Datum Newlyn (sea level) for height.

As these are two different systems, there is likely to be differences, but GNSS receivers can correct the difference between the two systems using a transformation. Please note: you will need to set your GNSS receiver's datum (the definition of the coordinate system used) to OSGB36 to correct differences. These corrections will allow your GNSS features to be represented in their true position on Ordnance Survey mapping.

The Ordnance Survey transformation between WGS84/ETRS89 and OSGB36/ODN is OSTN15/OSGM15. It is available online here and as software to download here. Developers wishing to use OSTN15/OSGM15 in software should see here.

What is GNSS jamming?

From time to time, GNSS signals may be subject to localised jamming. Jamming is the inability to ‘see’ GNSS satellites, which will affect functionality. OFCOM provide a free subscription service giving notice of any planned GNSS jamming exercises, so you can plan your GNSS activities without interruption.

What is the OS Net network and why is it so important?

The Ordnance Survey OS Net® network is the infrastructure which gives access to our national coordinate system in Great Britain.

The network consists of approximately 110 continuously operating GNSS receivers throughout Great Britain that stream GNSS data in real time to a central server.

OS Net makes the accurate determination of national standard coordinates much easier and more efficient for users who require accurate positioning, compared to traditional (pre-GNSS) methods. It is now possible to determine precise ETRS89 coordinates (European Terrestrial Reference System – an accurate, stable realisation of WGS84 for Europe) for your GNSS control stations with a single GNSS receiver, without ever leaving the site. These coordinates can be instantly and precisely transformed to OSGB36 National Grid and Ordnance Datum Newlyn height coordinates (or to a project-specific mapping grid if appropriate).

This technology supports an extensive range of mapping, engineering and environmental projects which would simply not be possible without it. Some 150 of our surveyors use it to collect field data for the production of our various map datasets.

Although OS Net as a service is not available commercially, our partners use our network data to develop and provide a basket of real-time and post-process GNSS correction services to customers within a wide range of markets. They decide their own end-user pricing policies.

What is the coordinate system for OS Net?

The OS Net coordinate system is ETRS89. OSGB36 National Grid and Ordnance Datum (OD) can be realised through a precise transformation and geoid model (see our Coordinate Transformer page).

For detailed information about the OS Net coordinate system and other in use in GB see our publication "A guide to coordinate systems in Great Britain" here.

Does OS Net support DNF principles?

Yes. We require that all use of OS Net services are coordinated to the primary reference frame (ETRS89). This means that all data collected through the use of OS Net uses a common framework and can therefore be shared.

What are RTK, dGPS and RINEX?

These are the three key positional services available via OS Net:

  • RTK (Real-Time Kinematic) GNSS gives accuracy at the few centimetre level in real-time.
  • DGNSS (Differential GNSS) gives sub-metre accuracy in real-time.
  • RINEX (Receiver INdependent EXchange Format) refers to data for post- process applications.

For fuller descriptions, please see the OS Net glossary.

What is a correction service?

A correction service seeks to rectify the majority of errors caused by the satellite orbits, clocks and the atmosphere. Correction services based on OS Net are provided by OS Net commercial partners. A list of current OS Net commercial services is here.

Where can I find out more information on surveying with free OS Net services?

Please refer to OS Net free services.

Can I access information from traditional control networks?
What level of accuracy is offered by your network of passive stations?

The standard error of the passive network is 0.055 m in plan and 0.066 m in ellipsoidal height at 95% confidence level.

Some OS Net services were ‘free to air’ through the GPS website. Will this continue now that commercial services are available?

Yes. A free service for post-process applications will be maintained (using readings every 30 seconds) and this will continue to be available. The page to order free RINEX data from OS Net stations is here.

How do I set my handheld receiver so that the displayed coordinates are compatible with Ordnance Survey mapping?

Your receiver will have a Datums setting to change the coordinates inside the receiver to output OSGB36 national Grid coordinates. If you have problems, your first point of contact should be your receiver manufacturer.

Why does my handheld GPS display a height of 40m when I know I'm at sea level?

The model of the Earth's surface used by many handheld GPS receivers is often a mean fit for the entire world. Because of this the model may not fit Great Britain very well resulting in a height discrepancy. Height is always the least reliable element in a GPS position fix, especially for a navigated position.

Why has my handheld GPS receiver not got a position fix after having been running for 10 minutes?

If your receiver is new or has not been used for some time it may take over 13 minutes to get an initial position fix. Your receiver uses an almanac containing information on the satellite constellation to search the sky for GPS satellites. If this almanac is out of date, it may take your receiver some time to lock on to an initial satellite and then a further 12.5 minutes for an updated version of the almanac to be transmitted to your receiver.

What is the difference between WGS84 and ETRS89?

Most people who are familiar with GPS have heard of the WGS84 (World Geodetic System 1984) coordinate system. This is a global coordinate system designed for use anywhere in the world. WGS84 coordinates are usually expressed as latitude, longitude and ellipsoid height.

WGS84 was designed for navigation applications, where the required accuracy is one metre or lower. A high-accuracy version of WGS84 known as ITRS (International Terrestrial Reference System) has been created in a number of versions since 1989, and this is suitable for international high-accuracy applications (it is used mostly by geoscientists). However, there is a problem with trying to use a global coordinate system for land surveying in a particular country or region. The problem is that the continents are constantly in motion with respect to each other, at rates of up to 12 centimetres per year. There are in reality no fixed points on Earth. In common with the rest of Europe, Great Britain is in motion with respect to the WGS84 coordinate system at a rate of about 2.5 centimetres per year. Over a decade, the WGS84 coordinates of any survey station in Britain change by a quarter of a metre due to this effect, which is unacceptable for precise survey purposes.

For this reason, the European Terrestrial Reference System 1989 (ETRS89) is used as the standard precise GPS coordinate system throughout Europe. ETRS89 is based on ITRS (the precise version of WGS84), except that it is tied to the European continent, and hence it is steadily moving away from the WGS84 coordinate system. In 2000, the difference between the ITRS (precise WGS84) coordinates of a point and the ETRS89 coordinates is about 25cm, and increasing by about 2.5 cm per year. The relationship between ITRS and ETRS89 is precisely defined at any point in time by a simple transformation published by the International Earth Rotation Service. There is a tool to transform between different ITRS realisations and ETRS89 realisations here.

The ETRS89 coordinate reference system is used as a standard for precise GPS surveying throughout Europe. Using ETRS89 you can ignore the effects of continental motion: to a high degree of accuracy, the ETRS89 coordinates of a survey station stay fixed, as long as there is no local movement of the survey station. ETRS89 has been officially adopted as a standard coordinate system for precise GPS surveying by most national mapping agencies in Europe.

Why should I never use OSGB36 triangulation points as GNSS control stations?

Before the launch of this web site, there was a lack of precisely coordinated and monitored GPS reference stations in Great Britain, and it could be expensive to buy their coordinates. This situation led, understandably, to many surveyors using OSGB36 triangulation stations as GPS control points, obtaining GPS (WGS84) coordinates of these stations by transforming the OSGB36 archive coordinates.

This web site changes this situation entirely. We make available, free of charge, RINEX data from the OS Net network of permanent GNSS stations. We do so because we are committed to encouraging best practice in GNSS surveying throughout Great Britain.

It is poor survey practice to use an OSGB36 triangulation point as a GNSS control station, because:

  • A triangulation station does not have accurate GNSS coordinates. All GNSS surveys should be based on control stations with accurate and recent ETRS89 coordinates. No such coordinates exist for OSGB36 triangulation stations.
  • A triangulation station is not maintained or monitored by Ordnance Survey for geodetic purposes. OS does not give any guarantees of the accuracy of archive OSGB36 coordinates, neither does it monitor the stability of OSGB36 monuments.
  • Most OSGB36 coordinates were determined before 1950, and the system has never been overhauled. The OSGB36 framework contains significant scale and orientation errors in a complex pattern, which can amount to 20m relative errors over long distances. Why degrade and distort your high-precision GNSS methods by using an outdated control framework?

For these reasons, only low-accuracy GPS coordinates (WGS84 coordinates) can be obtained from triangulation monuments. Therefore, although your resulting GNSS survey may or may not have good internal consistency (that is, high relative accuracy), it will not be consistent with other GNSS surveys in adjoining areas - that is, it will have low absolute accuracy. This effectively throws away one of the big advantages of GNSS surveying, which is the ability to achieve both high relative accuracy and high absolute accuracy at little or no additional cost. The next section outlines how to achieve this.

What is the best method for achieving high absolute accuracy in a GNSS survey?

All GNSS surveys should be based on accurate recent coordinates, in the ETRS89 coordinate system, of regularly monitored, positionally stable geodetic monuments. The OS Net stations are ideal for this, and are freely available. OS Net stations are continuously monitored on a daily basis and have ETRS89 coordinates with (typically) 5 mm horizontal accuracy.

Using the OS Net reference stations, ETRS89 coordinates of your own primary survey stations, and hence of all your surveyed points, can be established using your usual GNSS surveying methods. ETRS89 is the European standard precise GNSS coordinate system, so your GNSS survey will be consistent with thousands of other independent surveys throughout Europe. ETRS89 is in turn precisely related to all other precise GNSS coordinate systems across the world, so your survey is ultimately tied in to the most accurate geodetic models of the whole earth. This is what we mean by absolute accuracy, and it has important practical benefits in terms of consistency and compatibility between datasets.

We recommend that you archive your survey coordinates in the form of ETRS89 latitude, longitude and ellipsoid height. If you need to convert these coordinates to British National Grid eastings and northings, this can be done to high precision using the Ordnance Survey national Grid Transformation OSTN15 (available on this website in the coordinate transformation service). If you need to use a job-specific local mapping grid, you can do this in your usual software by defining appropriate map projection parameters. However, we recommend that you retain the archive of ETRS89 geodetic coordinates, so you can go back to them if required. If you need orthometric heights (mean sea level heights) of your survey stations, these are obtained using the Ordnance Survey National Geoid Model OSGM15 (available on this web site in the coordinate transformation service).

Is the National Grid Transformation OSTN15 really as accurate as using OSGB36 control stations?

Let's take the National Grid Transformation OSTN15 first. This converts ETRS89 GNSS coordinates to British National Grid coordinates, and vice versa. It is a complex transformation in that the parameters it applies vary in a complex way depending on your location in Great Britain. OSTN15 defines the National Grid such that: ETRS89 (from OS Net) + OSTN15 = true National Grid.

In discussing the accuracy of OSTN15, it's very important to understand exactly what we mean. The reason the OSTN15 transformation is needed is because the archived triangulation OSGB36, on which the National Grid is based, contains a complex pattern of distortions.

While it's true that two adjacent OSGB36 triangulation stations are usually in agreement with each other to within a few centimetres, if we compare two triangulation stations 10 km apart by measuring a precise GNSS vector, we will typically find an error of several decimetres in their relative archive coordinates. If the two stations are 100 km apart, the relative error between them might be four metres. If the stations are at opposite ends of the country, we will find a twenty metre relative error between the two, based on their archive OSGB36 coordinates.

The OSTN15 transformation precisely models these distortions in OSGB36. So the first point to bear in mind is that converting a precise GNSS survey to National Grid coordinates will always degrade its relative accuracy. A rule of thumb guide to the maximum magnitude of this distortion is 4 centimetres per kilometre. If this distortion is unacceptable for your application, do not use National Grid coordinates. Instead, work in ETRS89 coordinates which have a greatly superior reference framework quality (less than 5 millimetres relative distortion across the whole of Great Britain).

National Grid coordinates should be used when the requirement of the survey has compatibility with Ordnance Survey mapping. The positional accuracy of Ordnance Survey large-scale mapping depends on the scale of mapping: the most accurate mapping is at 1:1250 scale. Detail features on this mapping are positioned to 0.5 metre accuracy (1 standard error) relative to the OSGB36 framework. Therefore, the transformation we use to convert GPS coordinates to National Grid coordinates must accomplish this conversion to better than half a metre accuracy.

The National Grid Transformation OSTN15 defines the National Grid and models the old OSGB36 triangulation station framework to an accuracy of 0.1 m (1 standard error). This means it is five times more accurate than the most accurate Ordnance Survey mapping, relative to the OSGB36 framework. It is therefore more than sufficient for all survey tasks where the requirement is to achieve compatibility with Ordnance Survey mapping.

Can I install my own height bench marks on the Newlyn datum?

Yes. The services available on this website allow you to compute high-accuracy orthometric heights (mean sea level heights) relative to the Ordnance Survey height datum without visiting any Ordnance Survey bench marks, and without knowing in advance the orthometric heights of the control stations you use. Ordnance Datum Newlyn (ODN) is the usual definition of mean sea level on the British mainland and some islands.

Any survey station with precise ETRS89 GNSS coordinates determined using the OS Net Network can be used as a height bench mark by obtaining the equivalent orthometric height from the Ordnance Survey National Geoid Model OSGM15 (this is available from this web site in the coordinate transformation service). It is advisable to use several such stations on the same site and level between them, to check the consistency of the orthometric heights. There is no need to occupy traditional Ordnance Survey bench marks by GNSS, unless you have a specific requirement to check the relationship between your GNSS survey and existing bench marks in the area (for example, if you have based previous height surveys on those bench marks and you need the new survey to fit exactly with the old).

However, be aware that obtaining precise heights by GNSS is more difficult than obtaining horizontal coordinates. We recommend the use of IGS (International GNSS Service) precise satellite orbits (ephemerides). These are available from the IGS website. Mixing antenna types can affect the height accuracy of GNSS surveys. It is important to tell your software the antenna phase centre offsets to apply. The main practical implication is that GNSS observation times must be longer to obtain precise heights than would be used for ordinary surveys. It is possible to obtain a GNSS height with an accuracy of 2 cm (1 standard error) using the OS Net stations, by recording four hours of continuous GNSS observations at a primary survey station. Shorter observation times will generally produce less accurate results. In many cases, this will be more cost-effective than running levelling loops through a series of Ordnance Survey bench marks to establish Newlyn heights of your stations. And with the GNSS method, you obtain the height of the station as it is today. Few Ordnance Survey bench marks have been checked more recently than the 1960s, so it is possible that their archive heights may in incorrect.

How accurate is the Ordnance Survey Geoid Model OSGM15? OSGM15 has an accuracy of 8mm rms (1 standard error) for mainland Britain and better than 3cm rms for other areas. This is a measure of the absolute accuracy or possible error introduced by the model. However, because the parameters which comprise the model change gradually, the relative error introduced by the model within a 10 km extent would at most be just a few millimetres, i.e. the applied shift will be almost the same across the area. This is a similar or better error profile to that obtained by high-quality national levelling, with the advantage that you do not have to rely on bench marks that may have last been heighted fifty years ago.

Do I need to take antenna phase centre offsets into account?

For the best accuracy – particularly in height – yes. Modelling the correct antenna phase centre offsets becomes important when using mixed antenna types. The OS Net Network contains a mix of antenna types.

What is RINEX format?

RINEX stands for Receiver INdependent EXchange and is a globally accepted standard format for GNSS data.

Most GNSS processing software will read in RINEX data as well as the manufacturer's proprietary binary format. This allows data from different manufacturer's receivers to be processed together which can be very difficult if data in only the proprietary binary format is available. Most GNSS receiver manufacturers should also supply a conversion utility to convert their binary format files to RINEX files.

All GNSS data from the OS Net stations is supplied in RINEX format to ensure maximum compatibility with all the different GNSS processing software that users may have.

There is a naming convention for RINEX files as follows: -

Filename = nnnndddf.yyt

Where nnnn =4 character station identifier.

ddd =3 digit day of year number (including leading zeros if necessary).

f = observation "session" within the day – a digit or character to uniquely identify the session of observations at that station on that day. E.g. hourly sessions are usually identified by letters a to x. Session a is 00:00-01:00 (GPS time), session b is 01:00-02:00, and so on until session x which is 23:00-24:00. By convention 0 (zero) is used to identify a 24 hour session.

yy =2 digit year number (including leading zeros if necessary).

t = file type identifier – o = observations
n = satellite positions ("n" for GPS navigation data, g for GLONASS navigation data)

E.g. filename LEED076a.16o is from the OS Net station LEED (located at Leeds), on day 76 of the year 2016 (16th of March 2016). It is the first hourly file (session "a" so from 00:00 to 01:00) from LEED on that day and it contains observation data.

RINEX is a text format and the files can be viewed with any basic text viewing software. There is always a header block containing information such as station name, antenna type, time of observations, etc. One of the records in the header is APPROX POSITION XYZ, which usually contains the approximate position (to around 10 m) in WGS84 of the antenna. Ordnance Survey uses this field to store the actual precise position of the antenna in the ETRS89 datum. A comment is added to the header stating that the coordinates are no longer approximate. In this way users can be sure that the very latest coordinates for an Ordnance Survey Active GPS Network station can always be found in the RINEX observation file header.

For detailed information on the RINEX format see the Resources section of the IGS web site under "Products | Formats".

How are OS Net base stations monitored?

The positional quality of every OS Net station is monitored constantly in real time by the OS Net software.

The real time computed coordinates are constantly compared with the station’s accepted ETRS89 coordinates (which are based on at least 2 weeks data) and the differences in East, North and Up directions are computed.

Alarm thresholds are set in the software so that if a station's real time coordinates significantly differ from the accepted coordinates an alarm message is sent to the software operators. In this case the station would be immediately removed from the network and the reason for the movement investigated.

How do I change my coordinates from Latitude and Longitude to grid Eastings and Northings or vice versa?

You must first determine if you require a conversion or a transformation. That is, are you changing coordinate systems?

For example, if you have GNSS derived coordinates (based on the WGS84 or ETRS89 coordinate system) and require Ordnance Survey National Grid coordinates (OSGB36 coordinate system) or vice versa, then you require a transformation. The Coordinate Transformer facility will perform this operation on either a single position or using a batch file for multiple coordinate input.

If you wish to stay in the same coordinate system and simply express coordinates in a different format then a conversion is required. For example, changing the format of your OSGB36 National Grid eastings and northings to latitudes and longitudes (still in OSGB36) or changing from GNSS derived Earth Centered Earth Fixed (ECEF) XYZ cartesian coordinates in WGS84 to latitude, longitude and ellipsoidal height (still in WGS84). Our coordinate calculations spreadsheet lets you make these conversions.

For more information on datums and coordinate systems and the formulae to carry out coordinate conversions and some simple transformations, see the online guide.

Why were the OS Net station coordinates updated in August 2016?

Prior to August 26th 2016 the OS Net coordinates were based on a ratified (by the IAG subcommission "EUREF") offical realisation of ETRS89 in GB known as "EUREF GB 2001". This set of OS Net coordinates is now known as "OS Net v2001". The "parent" ITRF of OS Net v2001 was ITRF97 at epoch 2001.553.

In 2009 a new EUREF campaign was ratified as an official realisation of ETRS89. The new campaign was required due to the loss of many EUREF GB 2001 stations. These stations were replaced by a new sub set of the OS Net network known as "GeoNet" consisting of 12 "zero order" geodetic stations. Ordnance Survey, Ordnance Survey Ireland and Land & Property Services Northern Ireland collaborated on the campaign to produce a homogenous realisation of ETRS89 for the whole region. The campaign was ratified as "EUREF IE/UK 2009" and in GB the parent ITRF is ITRF97 at epoch 2009.756. So, OS Net coordinates related to this ETRS89 realisation are known as "OS Net v2009".

Between 2009 and 2016 other work took place to improve the OSGM geoid model related to the OS Net v2009 - see the FAQ "Why was the OSTN02/OSGM02 transformation updated to OSTN15/OSGM15 in August 2016?". Also when the OS Net network was complete the opportunity was taken to compute coordinates relative to EUREF IE/UK 2009 as a single figure for the first time. All this work was complete in 2016 and the new coordinates (and transformation models) went live on 26th August 2016.

The OS Net coordinate file containing v2009 and v2001 coordinates can be found here. A file of coordinate differences between v2001 and v2009 is here.

Why was the OSTN02/OSGM02 transformation updated to OSTN15/OSGM15 in August 2016?

Improvements in the coordinates (see the FAQ "Why were the OS Net station coordinates updated in August 2016?") and especially ellipsoidal heights of OS Net stations for the realisation of the ETRS89 system required a revision of the OSGM model. Also improved gravity data (since OSGM02) has enabled a more accurate model to be computed.

Also the fitting of OSGM02 to local datums in the Scottish islands (e.g. Outer Hebrides) was not optimal – this has been improved in OSGM15.

The OSTN15 transformation grid has been updated to take into account a slight change in the realisation of ETRS89 in GB.

If you have coordinates in OSGB36 which were transformed using the previous transformation (OSTN02/OSGM02) and wish to relate them to coordinates transformed using the new transformation (OSTN15/OSGM15) they should first be back-transformed into ETRS89 using OSTN02/OSGM02 and then forward transformed again into OSGB36 using the new transformation OSTN15/OSGM15. There is a tool on the transformation page to do this.

Users who have archived their coordinates in ETRS89 will not have to back transform to relate old and new datasets. The archive coordinates can be simply transformed again using the new OSTN15/OSGM15 transformation models. To achieve highest accuracy and take into account the small changes in OS Net coordinates (see the FAQ "Why were the OS Net station coordinates updated in August 2016?") the differences are give in this file here.

How do I calculate the azimuth (true bearing) between two points?

We offer a coordinate calculations spreadsheet that contains many utility functions for working with coordinates including azimuth calculations, calculating convergence, projection functions and t-T correction.

How do I calculate the difference between Grid North and True North (Convergence) at a location?

There is a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, projection functions and t-T correction.

How do I calculate the local scale factor?

There is a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, projection functions and t-T correction.

How do I calculate the t-T correction?

There is a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, projection functions and t-T correction.

What coordinate system is used in Ireland and other parts of Europe?

The Information and Service System for European Coordinate Reference Systems website at www.crs-geo.eu can help.

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