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

[Andrew C Aitchison]

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