[gdal-dev] Is it possible to write custom CRS WKTs within a GeoTiff?

[Even Rouault]

Le 09/01/2025 à 14:29, Paul Harwood a écrit :
>
>
>>     "If you can sacrifice the ellipsoidal height axis to avoid using
>>     a Projected 3D CRS, which is a bit of a geodetic dubious object
>>     (don't ask me more, but minds more enlightned than me question
>>     the soundness of using an ellipsoidal heights with a projected CRS)”
>
>     I won’t ask you more :) but maybe this can be discussed further
>     with those enlightened minds, perhaps on PROJ mailing list or a GH
>     issue?
>
> I would also be very interested in the background to this comment - in 
> a different thread.
If I remember well:  the issue comes from the fact that the axis 
definition for a projected CRS is a Cartesian coordinate system, which 
is fine for the 2D part of it. But when you add the ellipsoidal height 
into that picture, the axis of the ellipsoidal height is perpendicular 
to the surface of the ellipsoid, but the surface of the ellipsoid is not 
(exactly) the plane of the projected CRS. Towards the projection center, 
if the scale factor is 1 or close to 1, then both surfaces can probably 
be considered as close enough, but the farther you go from the 
projection center, the less this approximation is valid.

-- 
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