[PROJ] Converting coordinates between different ellipsoids... possible? valid?

[Kristian Evers]

Nyall,

This is basically the same question as you asked a few months ago:
https://lists.osgeo.org/pipermail/proj/2019-March/008390.html

Not sure exactly what celestial body you are trying to map this time,
but the problem remains the same: WGS84 is a CRS belonging to Earth,
that can’t meaningfully be transformed to a CRS on another
planet/moon/asteroid/whatever.

You can convert between different ellipsoids but does it make sense when one
has a radius 3 times larger than the other? Anyway, here’s how:

echo 12 56 0 0 | cct +proj=pipeline +step +proj=cart +ellps=WGS84 +step +proj=cart +R=2632345 +inv
12.0000000000   55.8213634673  3731132.8860        0.0000

Notice the height of the original point flying 3700 km above the ground on
the new ellipsoid.

Can you explain in which scenarios you need to do the transformation
you are asking for? That might make it easier to advise you.

/Kristian

On 20 May 2019, at 05:15, Nyall Dawson <nyall.dawson at gmail.com<mailto:nyall.dawson at gmail.com>> wrote:

Hey list,

If I attempt to create a transform using proj_create_crs_to_crs, with
the source proj string "+proj=longlat +datum=WGS84 +no_defs" and dest
proj string "+proj=longlat +a=2632345 +no_defs", I get the error
"internal_proj_create_operations: Source and target ellipsoid do not
belong to the same celestial body".

My question is -- should this be allowed? It worked ok in previous
proj versions, and it's a common situation in QGIS which I'm having
issues fixing after the v6 port.

Or is it violating some rule of transformation? How can I reproduce
the proj 4 behaviour here?

Cheers,
Nyall
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