[gdal-dev] Transformations with same horizontal datum but different geoid outputs the same Z value

[Paul Harwood]

I had to respond just because ...

GPS did not overthrow the Airy ellipsoid! It is alive and well in BNG where
it should be.

There are also organizations (like, I believe, MOLA) that prefer to use
LatLng with the Airy ellipsoid. I have a problem with that since it is
confusing (there is no *simple* notation in LatLng coords to tell you which
reference is used) and if you get it wrong the error in position is about
100m! Too big to be ignored but often too small to be obvious!

The response you received was because you mixed up ellipsoids and geoids.
4326 does not reference any geoid - it is a 2D reference system. You
probably meant the equivalent 3D CRS 4979 but that refers to ellipsoidal
height (above, as you say, the wgs84 ellipsoid ensemble). You talked about
the geoid, which is a different reference.

The Airy ellipsoid is not a Geoid. The OS Geoid is, I believe, OSGM15.

As far as converting from ellipsoidal height to orthometric height (i.e.
referenced to the Geoid) - I am not sure that it matters that GPS is using
WGS84 references. You still have to do a similar transformation and I would
expect that the final accuracy would be about the same.

On Mon, 17 Oct 2022 at 19:06, Andrew C Aitchison <andrew at aitchison.me.uk>
wrote:

> On Mon, 17 Oct 2022, Greg Troxel wrote:
>
> >
> > Andrew C Aitchison <andrew at aitchison.me.uk> writes:
> >> I do know that 4326 uses a geoid which is less accurate *over Great
> >> Britain* than the one the Great Britain Ordnance Survey have been
> >> using for a century or two (OSGB36 Datum 1936, Airy Spheriod 1830!).
> >> I suspect that other national geo-organisations do the same thing and
> >> you will lose the extra accuracy in standardising on one coordinate
> >> system over Europe.
> >
> > I don't follow this at all.  EPSG:4326 is a 2D system, so it doesn't
> > even have height, and has no associated geoid.
>
> Ah. I misremembered; I should have said the GRS80 ellipsoid:
>
>
> https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid#Compatibility_with_related_systems
>
> The Airy ellipsoid is a regional best fit for Britain; more modern mapping
> tends to use the GRS80 ellipsoid used by the Global Positioning System
> (the Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter
> than the GRS80 ellipsoid, and to be slightly less flattened).
>
> I do remember mourning that GPS overthrew the Airy Spheroid.
>
> --
> Andrew C. Aitchison                      Kendal, UK
>                     andrew at aitchison.me.uk
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