2
Aug
2016
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Is Britain on the move?

OS Maps uses GPS to position you

OS Maps uses GPS to position you

Australia has recently announced a 1.8m shift in its mapping coordinates, to compensate for the country’s 7.5cm shift north each year.  Inevitably the question is why, and could the same thing happen here?

In Australia, the shift is to take into account the growing difference between maps (and the coordinate reference system they’re based on) and the system used by satellite positioning (GPS). It’s a fact that the world is constantly shifting on tectonic plates, but maps (and their users) like fixed coordinates that don’t change. Before GPS, this was simple to achieve as most positioning and mapping was created from fixed ground points in a coordinate reference system tuned to a particular country. In Great Britain our fixed points included the very familiar trig pillars and we have a mapping coordinate reference system called OSGB36 National Grid which is fitted closely to our little bit of the Earth. Tectonic plate movements had little or no impact on the mapping coordinates or fixed points because they all moved “as one” and generally stayed the same shape. 

GPS is a global system and requires a coordinate reference system that has a good average fit to the whole Earth, and also has to change to keep up with the shifting of tectonic plates – it is effectively 4D (3D + time). The GPS coordinate reference system is WGS84 and is perfect for GPS, but perhaps not ideal for large- or mid-scale mapping because it moves (albeit slowly). Also for WGS84 the fixed point is not trig pillars on the ground, but satellites in orbit at approximately 20,000km and travelling at approximately 14,000kmph!

The differences between regional systems, such as OSGB36 and WGS84 means that GPS-derived coordinates have to go through a mathematical operation, known as a transformation, in order to fit them correctly on the map. Australia’s mapping system was updated in 1994, and fixed to the ground at that time, to be directly compatible with WGS84. However time and tectonic plate movement have taken their toll, and the mapping system and WGS84 have now drifted apart to a level that might be noticeable to a wider group of users other than geodesists and geologists. Hence everything now being dragged 1.8m to the north!

So, what’s the position (forgive the pun) in Great Britain?

OS Net locations across Britain

OS Net locations across Britain

The situation for us (and most of Europe) is not so bad. Europe’s GPS compatible datum, ETRS89, is fixed to the European tectonic plate at the time 1 January 1989 and moves by around 2.5cm each year. In theory, GPS-derived coordinates are now about 70cm away from where they should be in the ETRS89 system.

For most consumer users the 70cm difference is swamped by the accuracy limits of their receiver (phone, satnav etc) which are only accurate to within a few metres. So the difference is not noticeable if you’re out walking, or driving around the country.

For professional users, like OS surveyors, a correction service is used with the GPS to get centimetre-level accuracy. We use OS Net. This means they are automatically positioned in ETRS89 via the correction service so the 70cm drift is accounted for. A transformation is then used to get from ETRS89 to OSGB36 National Grid to be compatible with OS maps.

Clearly though, the problem of WGS84 drift relative to the mapping system is not unique to Australia and will eventually effect all countries. The drift happens at a constant rate but the increased use of GPS, plus other satellite positioning systems and the associated receivers becoming more accurate, means that the drift is likely to become more noticeable to more users.

What’s the answer?

There are a number of options. Mapping in each country could stay on a “local” system such as OSGB36, then an additional transformation could be employed to first go from WGS84 to ETRS89 and then on to OSGB36. Or all mapping could be based on ETRS89 and simply keep the time evolving WGS84/ETRS89 transformation. Or, transform all mapping to a WGS84 compatible system (at a certain point in time) and have coordinate jumps every few decades to keep up with WGS84. This is not an ideal answer and it seems Australia are only doing this once before implementing another system in 2020.

An alternative might be to have a dynamic “4D” coordinate system that constantly evolves like WGS84 and users become used to all coordinates and map data having a time stamp plus the availability of a transformation to bring coordinates into a common time frame if so desired.

For now, Ordnance Survey, and other mapping agencies in Europe, are all planning for the future when this becomes an issue and will be considering, in consultation with users, the best way to deal with it.

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6 Responses

  1. I’ve recently been in discussion with NSPS & xyHt where dynamic coordinate systems are becoming more of a common conversation. The principle is ideal though how they could be implemented into current geospatial systems so that they are kept current is an interesting question.
    Thanks for this post, it provides great clarity for the average geospatial user.

  2. Mark,
    Great article and explanation of the changing landscape. I recall working with you on my thesis project on the UK’s active GPS network years ago, and it looks like many of the issues addressed then are coming to fruition with the expansion of highly accurate navigation technology.

  3. My view is that in the UK we should continue to use OSGB36 locally and it should be maintained as a first class datum (i.e. restore the original triangulation stations!). Unfortunately in many (all?) GIS systems it is impossible to do accurate transformations between OSGB36 and WGS84. Whilst you can use OSTN02 to go from OSGB36 to ETRS89, you need an additional step to transform to/from WGS84, which also depends on a time parameter.

    As a footnote, we may also want to re-survey the original triangulation markers to take into account the small fluctuations that occur within our landmass. OSTN02 should be updated periodically to account for this.

    1. Hi Chris

      OSGB36 will be retained as the mapping coordinate system of GB for the foreseeable future. However, although still perfectly suitable for mapping, it could be considered that it is not longer “first class”. An amazing achievement at the time OSGB36 is now decidedly “legacy” when compared to modern, GPS derived, high accuracy, global coordinate systems. At some point in the future users of OSGB36 will be consulted about a possible move to an ETRS89 based mapping system. Clearly though this will be a long and considered process.

      Restoring the original triangulation stations is not a practical option. “Always on” networks of permanent GPS stations such as OS Net offer un-paralleled accuracy and monitoring of small fluctuations of the landmass (known as intra-plate velocities), in real time, that legacy networks of ground based markers could never match.

      OSTN02 has very recently been updated to OSTN15. This is due to OS Net station coordinates being updated to an improved realisation of ETRS89 and OSTN02 being tweaked to minimise the impact on resultant OSGB36 coordinates.

      Thanks, Mark

  4. Warren

    What no one seems to have addressed is the everyday use of the ordnance survey grid to define property boundaries, despite the warnings given that the result cannot be accurate. Comparing a land conveyance of 100 years ago with modern positioning throws up an error of 2.5 meters. Surveyors should apply a correction to show the true position of a property, but I have never heard of one doing so. It is very difficult to convince the lay person that the National Grid is continuously moving.

    1. Hi Warren

      In terms of property boundaries, that responsibility and expertise lies with Land Registry. There is more information on that, and the OS role, on our website, along with contact details for LR: https://www.ordnancesurvey.co.uk/resources/property-boundaries-owners.html

      Regarding the National Grid continuously moving, it actually isn’t. The WGS84 coordinate system for GPS is the one that is moving (in a global sense) to keep track of tectonic plate changes. The National Grid, relative to our mapping detail, is stable and fixed. A difference between a 100 year old map and a correctly fixed modern National Grid position will be down to minor inaccuracies in the original survey and mapping datum (OSGB36 National Grid v “County Series”) rather than changing global datums.

      Kind regards
      Gemma

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