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

[Lesparre, Jochem]

True. However, a M10 nut for a car is a bit to generic than an ellipsoid for a celestial body. It would be more appropriate to compare a ellipsoid with a paint colour. You can paint a door of an Volkswagen with a paint from Ford, but that door won’t be matching with any existing car.

Regards, Jochem


From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Thomas Knudsen
Sent: dinsdag 25 mei 2021 10:27
To: Nyall Dawson <nyall.dawson at gmail.com>
Cc: PROJ <proj at lists.osgeo.org>
Subject: Re: [PROJ] Determine if same celestial body, prior to 8.1

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<mailto: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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