Transformations to OSGB36 and WGS84 in FME

FME from Safe Software is very powerful and has a lot of built in functionality for coordinate and datum transformations [4]. However, it is a bit obscure in how it actually does the transformations and what parameters it uses and how to change them.

The default transformation used in FME to convert from EPSG: 27700 to WGS84 is a 7-Parameter transformation with the following parameters:

 DX: 446 DY: -99 DZ: 544 Scale: -20.8927 rX: -0.945 rY: -0.261 rz: -0.435

This transformation is defined by the NGA (National Geospatial Agency) and is used in the US military transformation program Geotrans [5]. It is also used in ArcGIS and is defined as the NGA_7PAR transformation.

However, it does not have an accuracy statement and is not the 7 parameter transformation recommended by Ordnance Survey.

FME will use this transformation when converting data from OSGB36 to WGS84 either explicitly or implicitly. For instance, if you choose to transform data to Google Earth KML it will implicitly do a transformation to WGS84, for instance when converting OS MasterMap data from PostGIS (with data in EPSG: 27700) and converting to KML.

It will also choose this transformation if you specify that the source coordinate system as the Esri WKT British National Grid projection.

Figure 2: FME Coordinate Systems

The relative accuracy of this transformation can be seen in the following example which shows the junction of Grange Road and Causewayside. The black data is OS MasterMap transformed to WGS 84 using the default FME conversion. The blue data is the same data transformed using OSTN02. A difference of approximately 1.8m can be observed,

Figure 3: Difference between FME default and OSTN02

Using OSTN02 in FME

To ensure that FME uses the more accurate OSTN02 transformation it is necessary to define the source coordinate system as: OSGB-GPS-2002.

Setting the source to this coordinate system will automatically ensure that on conversion to WGS84 (or ETRS89) it will use the more accurate OSTN02 transformation as recommended by Ordnance Survey [2].

Defining your own coordinate system in FME

You can define your own transformation (for instance defining the 7 Parameter Petroleum Shift) in FME but you need to edit some files to do this. Details can be found at [6]:

The FME Coordinate System Gallery (Tools à Browse Coordinate System) shows the list of coordinate systems available and in the Description filed what transformations it will use.

Summary

For most applications the default transformation is probably acceptable, however with no accuracy statement associated with the NGA transformation it is difficult to tell. If absolute accuracy is not required then the 7 parameter ‘Petroleum Shift’ should be used (but needs to be defined in FME).

However, if transforming OS MasterMap data, or any data where high accuracy is important using FME, you should ensure that the OSTN02 transformation is being used by defining the source coordinate system as OSGB-GPS-2002.

References

[1] eLearning module on Coordinate systems and projections: http://digimap.edina.ac.uk/webhelp/training/training.htm#elearning/eLearning.htm

[2] Ordnance Survey – A Guide to coordinate systems in Great Britain

http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf

[3] Seazone: Introduction to Coordinate Reference Systems

http://www.seazone.com/help-support/SeaZone_Introduction-to-coordinate-reference-systems.pdf

[4] Coordinate Systems in FME: https://knowledge.safe.com/articles/585/using-coordinate-systems-within-fme.html

[5]Geotrans Geographic Translator

http://earth-info.nga.mil/GandG/geotrans/

[6]Defining Coordinate Systems in FME: http://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Coordinate_Systems/Home_cs.htm

[7]Esri UK – Understanding which Transformation to Choose in ArcGIS: http://communityhub.esriuk.com/journal/2012/3/26/coordinate-systems-and-projections-for-beginners.html

[8] Esri UK – What are my OGC web services displaced by about 120m?:  http://communityhub.esriuk.com/journal/2012/10/12/why-are-my-ogc-web-services-displaced-by-about-120-metres.html

[9] Other information about implementing OSTN02 in PostGIS directly is available here:

http://blog.mackerron.com/2012/07/03/ostn02-for-postgis/

[10] Information about transformations in QGIS

http://gis.stackexchange.com/questions/76411/issues-using-geographic-and-projected-coordinate-systems-qgis-2-0

[11] Blog from Frank Warmerdam indicating how transformations are chosen in proj.4

http://fwarmerdam.blogspot.co.uk/2010/03/in-last-few-weeks-i-believe-i-have-made.html

[12] PROJ.4 General Parameters

http://trac.osgeo.org/proj/wiki/GenParms#nadgrids-GridBasedDatumAdjustments