[PROJ] Determine if same celestial body, prior to 8.1

[Thomas Knudsen]

Not exactly trying to answer your question, but I will point out that the
setting is slightly dubious, since you can use any ellipsoid for any
celestial body (although with results as deserved :-)). The important thing
is the datum/reference frame, not really the ellipsoid: While classical 2D
datums are closely tied to an ellipsoid, modern 3D reference frames are
based on cartesian coordinates, and the ellipsoid is purely an
afterthought, necessary only for conversion to "geo"graphical coordinates.

So while "most useful" for celestial bodies with a physical surface
resembling the ellipsoid at hand, in reality you can use any ellipsoid for
any body (although your latitudes, longitudes, and ellipsoidal heights may
look counterintuitive if using a mal-fitting ellipsoid). So having a
"celestialBody" property on an ellipsoid is a bit like attaching a brand
name to a generic component (e.g. "a 'Volkswagen' M10 nut")

Thomas


Den tir. 25. maj 2021 kl. 06.14 skrev Nyall Dawson <nyall.dawson at gmail.com>:

> Hi list,
>
> I'm wondering if there's any way in proj versions < 8.1 to determine
> (from the C api) whether two ellipsoid objects refer to the same
> celestial body?
>
> Ellipsoid::celestialBody() wasn't exposed to C api prior to 8.1, and
> that's the method used by proj internally in
> createOperationsGeodToGeod to determine whether two ellipsoids belong
> to the same body.
>
> Nyall
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