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 Compass (China) and Galileo (Europe) satellite navigation systems.

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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.

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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.

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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.)

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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. 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.

Accurate offline transformations between WGS84/ETRS89 and OSGB36 are also available.

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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.

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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.

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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 pages).

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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.

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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.

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A correction service seeks to rectify the majority of errors caused by the satellite orbits, clocks and the atmosphere.

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Please refer to OS Net free services.

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Yes. We provide the following resources:

Passive GNSS station database

Vertical control (bench marks)

Liverpool - Newlyn datum differences

Triangulation stations

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

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Yes. A free service for post-process applications will be maintained (using readings every 30 seconds) and this will continue to be available.

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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.

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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.

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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.

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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.

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.

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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.

The launch of this web site changes this situation entirely. Ordnance Survey now give you their entire National GPS Network of active and passive GPS control stations, at no cost, directly from this web site. We have done this because we are committed to encouraging best practice in GPS surveying throughout Great Britain.

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

• A triangulation station does not have accurate GPS coordinates. All GPS 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 (with the exception of the clearly marked trig pillars which have been re used as passive GPS stations).
• 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 20 m relative errors over long distances. Why degrade and distort your high-precision GPS 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 GPS survey may or may not have good internal consistency (that is, high relative accuracy), it will not be consistent with other GPS surveys in adjoining areas - that is, it will have low absolute accuracy. This effectively throws away one of the big advantages of GPS 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.

Note: perhaps confusingly, some OSGB36 triangulation pillars have been reused as passive GPS stations, simply because they are convenient publicly-accessible monuments. These stations can be identified by a metal plaque bearing the words 'This monument forms part of the Ordnance Survey National GPS Network'. The comments above do not apply to these stations.

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All GPS surveys should be based on accurate recent coordinates, in the EtrS89 coordinate system, of regularly monitored, positionally stable geodetic monuments. The active and passive stations of the OS National GPS Network are ideal for this, and are freely available from this web site. Our active stations are continuously monitored on a daily basis and have ETRS89 coordinates with (typically) 5 mm horizontal accuracy. Our passive stations are monitored on a five year cyclic programme, and have ETRS89 coordinates with (typically) 5 cm horizontal accuracy (better for some stations).

Using the National GPS Network reference stations, ETRS89 coordinates of your own primary survey stations, and hence of all your surveyed points, can be established using your usual GPS surveying methods. ETRS89 is the European standard precise GPS coordinate system, so your GPS survey will be consistent with thousands of other independent surveys throughout Europe. ETRS89 is in turn precisely related to all other precise GPS 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 OSTN02 (available on this web site in the Coordinate converter 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 OSGM02 (available on this web site in the Coordinate converter service).

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Let's take the National Grid Transformation OSTN02 first. This converts ETRS89 GPS 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. OSTN02 now defines the National Grid such that: ETRS89 (from National GPS Network) + OSTN02 = true National Grid.

In discussing the accuracy of OSTN02, it's very important to understand exactly what we mean. The reason the OSTN02 transformation is needed is because the archived triangulation OSGB36, on which the National Grid is based, contains a complex pattern of distortions.

While it is 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 GPS 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 OSTN02 transformation precisely models these distortions in OSGB36. So the first point to bear in mind is that converting a precise GPS 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 OSTN02 now 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.

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Yes. The services available on this web site 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 GPS coordinates determined using the National GPS Network can be used as a height bench mark by obtaining the equivalent orthometric height from the Ordnance Survey National Geoid Model OSGM02 (this is available from this web site in the Coordinate converter' 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 GPS, unless you have a specific requirement to check the relationship between your GPS 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 GPS is more difficult than obtaining horizontal coordinates. We recommend the use of IGS (International GPS Service) precise satellite orbits (ephemerides) rather than the RINEX navigation files supplied with the active station data (which contain the GPS broadcast satellite orbits). These are available from the IGS website. Mixing antenna types can affect the height accuracy of GPS surveys. It is important to tell your software the antenna phase centre offsets to apply. The main practical implication is that GPS observation times must be longer to obtain precise heights than would be used for ordinary surveys. It is possible to obtain a GPS height with an accuracy of 2 cm (1 standard error) using the National GPS Network active stations, by recording four hours of continuous GPS 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 GPS method, you obtain the height of the station as it is today. Few Ordnance Survey bench marks have been checked="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 OSGM02? OSGM02 has an accuracy of 2 cm rms (1 standard error) for mainland Britain and better than 4 cms 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.

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We recommend that the active stations are always used for high-accuracy heighting work, rather than the passive stations. The GPS observation period should be as long as possible: in our tests, typical 1.5 cm ellipsoid height accuracy (one standard error) was obtained from a 4 hour GPS observation, and using advanced scientific GPS processing software. To obtain the best possible heighting accuracy of about 1.0 cm (one standard error), careful site selection, geodetic quality GPS equipment, scientific analysis software, and a 24hr observation period are recommended. It is also important that antenna phase centre offsets are modelled correctly.

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In the active station RINEX data files. The fields in the RINEX header marked 'approx coords' contain the official precise coordinates of the station. There is a comment to this effect in the RINEX header. The active stations are precisely monitored on a daily basis, and we may change these coordinates from time to time if we detect movement in a station.

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

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The letter prefix in the Passive Station numbering system is largely a historical one indicating the origins of the station.

Stations prefixed with the letter 'B' are stations originally included in the SciNet (EUREF GB92) network of stations. Before the Active GPS Network these were of a higher order, however there is no longer any difference in quality between any of the designated Passive Layer stations.

Stations prefixed with the letter 'C' are stations which originally met the criteria for a National Network GPS (Passive Layer) station. Namely: have good aspect of the sky for GPS occupation; be accessible 24 hours a day to a two-wheel drive vehicle in all weather conditions; and permission for access, if required, easy to obtain. Most of these monuments were constructed specifically for GPS use, though where suitable, existing Fundamental Bench Mark or Triangulation monuments were adopted.

Stations prefixed with the letter 'H' or 'T' are Fundamental Bench mark or Triangulation monuments included in the Passive Network for transformation computation purposes, but do not generally meet the access requirements for a 'C' designated station.

The number immediately following these prefixes differentiates between stations when more than one exists within the same km square. Eg C2SZ3090, a station created in the km square SZ3090 to replace C1SZ3090 which has been destroyed.

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RINEX stands for Receiver Independent Exchange and is a globally accepted standard format for GPS data.

Most GPS 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 GPS manufacturers should also supply a conversion utility to convert their binary format files to RINEX files.

All GPS data from the Ordnance Survey Active GPS Network stations is supplied in RINEX format to ensure maximum compatibility with all the different GPS processing software that users may have.

RINEX files usually come in pairs - a file of GPS observations (the "o" file) and a file of corresponding satellite positions (the "n" file). Generally GPS processing software will require both "o" and "n" files to be present in order to read in the RINEX data. 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 =file sequence number within the day – a digit or character to uniquely identify the session of observations at that station on that day. E.g. observations between 09:00 and 11:00 could be session "1" and between 15:00 and 17:00 on the same day could be "2". 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 navigation data)

E.g. filename LEED0761.01o is from the station LEED (OS active station located at Leeds), on day 76 of the year 2001 (17th of March 2001). It is the first file from LEED on that day and it contains observation data. LEED0761.01n would be the corresponding satellite positions file.

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, a link to a text file of the latest specification is usually available in the FAQ's section of the IGS web site - http://igscb.jpl.nasa.gov/faqs.html

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The positional quality of every Active Network 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.

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The MARKER NUMBER field contains a unique ID number for the GPS antenna site. E.g. the active station at Leeds (LEED) has the number 13215S001. These numbers are known as DOMES numbers and are issued by the International Earth Rotation Service (IERS). The IERS maintains a worldwide database of permanent GPS sites using DOMES numbers. The numbers are especially important for distinguishing between multiple antennae on the same site. E.g. if there was more than one antenna at Leeds the general site ID would still be LEED but each antenna would be identified by it's unique DOMES number.

A sub network of the Active stations has been accepted as an official extension of the EUREF coordinate reference frame (see "Why were the Active Network station coordinates updated in February 2002?"). One of the terms of acceptance is that the stations have DOMES numbers.

DOMES numbers are unique and the 4 character site ID should be unique also. Before the submission to IERS a cross check was done against European and world wide stations known to Ordnance Survey, and a clash was found for the station in Mallaig. It's ID at the time - MALL, clashed with one already in use for a station in Mallorca, so Mallaig was changed to MALA. When IERS returned the DOMES numbers they informed us that they had found a further clash in their more extensive database and changed Mallaig from MALA to MALG. This is why Mallaig has gone through 2 name changes in a short space of time.

The MARKER NUMBER field is simply filled with a repeat of the station's 4 character ID.

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The accepted GPS coordinate system through out Europe is the European Terrestrial Reference System 1989 (ETRS89), which is fixed in time at the epoch 1989.0. Europe also has an accepted coordinate reference frame, within ETRS89, known as EUREF. EUREF is controlled and monitored by the International Association of Geodesy (IAG) sub-commission for Europe (who also call themselves EUREF).

The Active station coordinates have always been computed to the highest possible standard and expressed in EtrS89 at epoch 1989.0. This makes them compatible with coordinates in the rest of Europe. However the official IAG extension to the EUREF frame in GB remained the 26 passive stations of the EUREF GB 92 network (also known as SciNet 92). This network was observed in 1992 but now, with the modern equipment and techniques of the Active network, a much better EUREF extension can be computed. Once the Active network was fully established it was decided to make a sub network of Active stations the new official extension to EUREF in GB.

Extensions to EUREF have to be computed from a specific observation campaign so, in July 2001, 2 weeks of continuous GPS data from the active network was used. This was supplemented by data from 6 International GPS Service (IGS) fiducial stations in Europe and a continuous week of data from 3 existing EUREF GB 92 stations (for comparison purposes).

The latest processing techniques (as recommended by EUREF) were used to achieve the highest possible accuracy. Although only a sub network is for the extension to EUREF all active stations were computed at the same time and are therefore of comparable accuracy. The resulting network (EUREF GB 2001) was submitted to the EUREF Technical Working Group at their October 2001 meeting.

The network was accepted by the TWG as the new extension to EUREF in GB. One of the provisions of acceptance was that the EUREF stations be assigned DOMES numbers (see separate FAQ - "What is the MARKER NUMBER in the RINEX header?"). These numbers were issued in January 2002.

The coordinates of the active stations were updated in February 2002 with the coordinates from the EUREF GB 2001 campaign. The change in coordinate values is small, being less than 1 centimetre in Earth centred cartesian X, Y and Z directions.

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The new and definitive Ordnance Survey transformation and geoid model OSTN02 / OSGM02 is an improvement over the previous models OSTN97 / OSGM91. The horizontal transformation accuracy has been improved from 0.2m to 0.1m rms and the vertical accuracy has been improved from 0.05m to 0.02m rms. This means that coordinates transformed using both new and old models are very likely to differ.

If you have coordinates in OSGB36 which were transformed using the previous transformation (OSTN97 / OSGM91) and wish to relate them to coordinates transformed using the new transformation (OSTN01 / OSGM02) they should first be back-transformed into ETRS89 using OSTN97 / OSGM91 and then forward transformed again into OSGB36 using the new transformation OSTN02 / OSGM02. An executable is available which will do this back transformation for users who no longer have access to OSTN97 / OSGM91. Download the executable file.

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 OSTN02 / OSGM02 transformation models.

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Coordinates of ALL the passive stations were updated following a complete readjustment of the entire network. This new adjustment is known as OSGPS2002. OSGPS2002 incorporates many extra observations and also extends the passive network into the Scottish Islands, the Scilly Isles and the Isle of Man.

The expected coordinate errors at a passive station are now improved to 0.055 m horizontal (95%) and 0.066 m vertical (95%). Hundreds of observations between passive and active stations have been included in OSGPS2002 meaning that the two networks are now in closer sympathy. The rmse East, North and Up residuals between OSGPS2002 and the previous adjustment are respectively 0.0208 m, 0.0118 m and 0.0416 m.

These changes are due to the improvements in the network through the extra observations and also due to the change in definition of EtrS89 via the fixed control. For the previous adjustment ETRS89 was defined by fixing the 26 EUREF GB 92 (also known as SciNet92) stations around the country. For OSGPS2002 a much improved definition of ETRS89 has been realised by fixing the active stations to their EUREF GB 2001 coordinates.

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An error was discovered in the antenna phase centre centre offsets used in the original EUREF GB 2001 campaign to compute the Active station coordinates. This resulted in errors in the station heights. The EUREF GB 2001 campaign was reprocessed with correct antenna offsets and the new version was ratified by a resolution of the annual EUREF symposium.

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As part of the significant changes to the Active GPS network that have happened since late July 2005, Ordnance Survey have now updated from version 2.0 to version 2.1 of the RINEX format for most GPS station's raw data files. The only change that a user may see is in the small header above each data block. Between each 2-digit satellite number there is now a letter 'G' which identifies that the satellite is a GPS satellite. This ability to differentiate between satellite systems has been in Version 2.0 since 1990, but either a space or a G could be used. The new system that Ordnance Survey uses to generate RINEX data now puts in the G.

The outcome of this should be non-existent to the vast majority of users. Some older software packages however may have trouble in reading these new RINEX files. The solution is to either get an updated version of software from your vender or to do a "Find / Replace" operation on the files - replacing the G's with spaces. Ordnance Survey do apologise for any inconvenience that this may cause to users.

>Example of old data block header

05 8 2 0 0 0.0000000 0 8 03 15 16 18 19 21 22 27

Example of new data block header

05 8 2 0 0 0.0000000 0 803G15G16G18G19G21G22G27

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You must first determine if you require a conversion or a transformation. I.e. are you changing coordinate systems?

For example if you have GPS 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. E.g. changing the format of your OSGB36 National Grid eastings and northings to latitudes and longitudes (still in OSGB36) or changing from GPS derived Earth Centered Earth Fixed (ECEF) XYZ cartesian coordinates in WGS84 to latitude, longitude and ellipsoidal height (still in WGS84). These conversions can be done using our coordinate calculations spreadsheet.

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

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

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

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

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

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

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Raw GPS positions have an accuracy of around 5 to 10 metres in plan, and 10 to 20 metres in height. This inaccuracy is caused by the errors in the GPS, summarised here:

OS Net corrects the orbital, atmospheric and satellite clock errors.

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The definitive Ordnance Survey transformation and geoid model OSTN02 / OSGM02 is an improvement over the previous models OSTN97 / OSGM91. The horizontal transformation accuracy has been improved from 0.2m to 0.1m rms and the vertical accuracy has been improved from 0.05m to 0.02m rms. This means that coordinates transformed using both new and old models are very likely to differ.

If you have coordinates in OSGB36 which were transformed using the previous transformation (OSTN97 / OSGM91) and wish to relate them to coordinates transformed using OSTN02 / OSGM02 they should first be back-transformed into ETRS89 using OSTN97 / OSGM91 and then forward transformed again into OSGB36 using OSTN02 / OSGM02. An executable is available which will do this reverse transformation for users who no longer have access to OSTN97 / OSGM91.

Download the executable file

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 OSTN02 / OSGM02 transformation models.

OSTN02 / OSGM02 transformation files

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