[Proj] U.K. OSTN02 and OSGM02

[Paul Kelly]

On Mon, 19 Dec 2005, Paul Kelly wrote:

> 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.

I'm so sorry I've just made that much more confusing that it needed to be! 
Of couse the geoid is nothing to do with the ellipsoid but models instead 
the gravitational pull of the earth, which is what determines the relation 
of different heights to the sea level-based vertical datums used in 
ordnance survey maps.

So I should have said OSGM02 relates the ETRS89 ellipsoidal height (which 
is what you can get from a GPS receiver) to the height above sea level (which
is determined by the geoid). And the vertical datum used in Northern Ireland
is indeed different from that used in Great Britain.

There are many people on this list far more qualified than me to explain 
this, but I hope at least I've been able to explain roughly what it means 
from a practical point of view.

Paul