GIS Tips – Don’t trust your GIS, have you heard of OSTN02?

You wouldn’t believe the number of GIS consultants. managers and technicians I have met who haven’t heard of OSTN02, what is more worrying some of them work in prominent roles and make some big decisions.

Okay, so if you don’t live in the United Kingdom or work with UK spatial data, then you can switch over now, though if you do live in the UK and work with UK spatial data in any form and you haven’t got a clue what I am rattling on about, read on.

What is OSTN02?

Ordnance Survey Transformation Network 02, OSTN02, is a geodetic transformation for  Ordnance Survey British National Grid….in plain English, it is a transformation for working between WGS84, ETRS89 & OSGB36.

Coordinate systems such as WGS84 & ETRS89 are Geographic whereas OSGB36 is Projected (image care of ESRI)

A quick catch up on British National Grid (It will help later)

The grid is based on the Ordnance Survey Great Britain 1936, OSGB36 datum (based on the Airy 1830 ellipsoid), and was introduced after the retriangulation of 1936–1962.

The Airy ellipsoid is a regional best fit for Britain; more modern mapping tends to use the GRS80 ellipsoid used by the GPS (the Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter than the GRS80 ellipsoid, and to be slightly less flattened).

Over the Airy ellipsoid a straight line grid, the National Grid, is placed with a new false origin (to eliminate negative numbers), creating a 700 km by 1300 km grid. This false origin is located south-west of the Isles of Scilly. The distortion created between the OS grid and the  projection is countered by a scale factor in the longitude to create two lines of longitude with zero distortion rather than one.

Random fact: OSGB 36 was used by UKHO for Admiralty Charts in UK waters until 2000, after this WGS84 was used!

Why you can’t trust your GIS

The most common geodetic transformation from WGS84 to a projected coordinate system (such as OSGB36) converting between  is called the Helmert datum transformation, this is a 7 parameter transformation which results in a typical 7m error from true.

It is at this point that you need to start up your ArcGIS or QGIS and remind yourself about the transformations which you are using. If you are using ArcGIS 3.2 – 10, you will most probably be using one of these Helmert transformations:

Table from ESRI geoXchange

If this isn’t enough evidence that you need to move to the OSTN02 transformation, then look at the map below by Steve Baker which shows the difference between OSTN02 and the ESRI recommended WGS84_to_OSGB36_Petroleum:

Convinced? So how does this OSTN02 work?

Rather than use the Helmert 7 parameter transformation, the OSTN02 uses hundreds of reference stations (OS Net) throughout the UK:

https://i0.wp.com/www.ordnancesurvey.co.uk/oswebsite/images/products/OSNetWebSiteMap_fullsize_pluscolours.jpg

OS Net stations 2012 thanks to Ordnance Survey

 Using the OS Net the transformation models the detailed distortions in the 1936–1962 retriangulation of OSG36, and achieves backwards compatibility in grid coordinates to the sub-metre accuracy.

For more detailed information, the Ordnance Survey has a wealth of free resources covering this: Read more here

Okay, very clever, but how can I use it?

Although the OSTN02 doesn’t ship with ArcGIS 3.1 – 10.1 (at time of printing), Ordnance Survey have worked very closely with ESRI UK to enable the transformation to be used, it can be downloaded free of charge from here: OSTN02 add-in for ArcGIS

To utilise this transformation method simply download and paste the OSTN02_NTv2.gsb into a folder called ‘C:\Program Files\ArcGIS\Desktop10.1\pedata\ntv2\uk’. You will need to create the folder called ‘uk’ which must be in lower case. Next time you restart ArcGIS 10.1 you will be able to use this transformation from the Geographic Coordinate Systems Transformations dialog box

This transformation is also available for QGIS through Ordnance Survey and can be downloaded from here: OSTN02 in NTV2 format

Around here I should post some instruction about how to install it, problem is that I haven’t been entirely successful in getting it to work, hopefully the amazing QGIS community will (as usual) point out some stupid spelling mistake I am making and I will be able to update this post shortly to provide a little user guide (it seems easy enough!!).

Nick D

2 thoughts on “GIS Tips – Don’t trust your GIS, have you heard of OSTN02?

  1. Very informative post Nick, I’ve been struggling with this in QGIS for a while & have ended up doing the WGS84 -> OSGB conversion with the conversion utility via the OS website, prior to which my surveyed points were all about 6-8m off. The QGIS plugin ‘Transformation Tools’ seems to be able handle this but some clearer explanation of how to set it up would be appreciated… for me at least! TimG

    1. Hi Tim,

      QGIS is relatively easy as the PROJ4 supports grid based datum adjustments (.grb files)…if you download the OSTN02 transformation (.grb file) from their website, I remember it is simply just a matter of copying it to the PROJ4 folder. I’m away from my GIS machine right now but will put something up when I am back in action 🙂

      There is some useful information on the .grb format and including it, here: http://trac.osgeo.org/proj/wiki/GenParms#nadgrids-GridBasedDatumAdjustments

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