[Proj] U.K. OSTN02 and OSGM02

Paul Kelly paul-grass at stjohnspoint.co.uk
Mon Dec 19 10:26:07 EST 2005

Hello Roger

On Mon, 19 Dec 2005, Roger Oberholtzer wrote:

> Is it possible to calculate the U.K.'s OSTN02 and OSGM02 X/Y coordinates
> with proj?

OSGM02 is the geoid model that models the separation between the
WGS84/ETRS89/GRS80 ellipsoid and the Airy ellipsoid used for Ordnance Survey
heights (or the Modified Airy ellipsoid in the case of Northern Ireland, which
OSGM02 is also supposed to apply to but didn't seem quite right when I tried
it about two years ago). PROJ.4 can't make use of this information and it 
would need to be applied to the WGS84 height separately from the 
re-projection step.

OSTN02 is the transformation grid to convert Ordnance Survey National Grid 
eastings and northings to/from ETRS89/WGS84/GRS80 latitude and longitude. It 
would be possible to use this data with PROJ.4 by converting it into the 
gsb format, which PROJ.4 recognises. I did something similar for the 
national gridshift file used in France (see 
http://www.stjohnspoint.co.uk/gis/france.htm although I have recently 
recognised that this might contain a VERY small error due to slightly 
different parameters used for the Clark 1880 ellipsoid as compared to the 
parameters built into PROJ.4) and would like to do it for OSTN02 but 
haven't had time.

Sorry I haven't really been any help.


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