[PROJ] Forcing Specific Transformation

[Peter Townsend]

I have a question about constructing specific transform operations between
two CRS's. If you pop over to EPSG.org and search "ED50 to WGS 84", you get
37 transforms back. I'd like to focus on one of them in particular, ED50 to
WGS84 (42) aka EPSG:15964.

I know the PROJ team has no control over epsg.io, but it lists 15964 as a
possible transform for epsg:23031 (https://epsg.io/23031-15964).

If I dump out the proj operations between 23031 and 4326, I don't see a
concat transform that uses (42).
projinfo.exe -k operation -s epsg:23031 -t epsg:4326 --spatial-test
intersects -o PROJ

As far as I know, I can't build a PJ object using the operation context
functions to get a pipeline string in order to build this transform. I
tried specifying the bbox for it to get more specific, but it still doesn't
show up.
projinfo.exe -k operation -s epsg:23031 -t epsg:4326 --spatial-test
intersects --bbox -13.87,34.91,-7.24,41.88 -o PROJ
(I get back (1), (13), (28), (17), (41), and ballpark.)

My problem is this. Suppose I get a request from someone who *really*
believes in the contents of epsg.io. And they want to do a transform that
appears on there but for some reason it doesn't appear in the proj
operation list. Maybe it's the cold hard truth that the transform isn't
technically possible, or there's a lookup bug or quirk. Doesn't really
matter in the end.

I did try proj_crs_create_bound_crs with 23031/4326/15964 as the
base/hub/transform accordingly and it did succeed in this case, but I'm not
sure if this is the best solution in general. I got back the PROJ.4
definition on the epsg.io page: "+proj=utm +zone=31 +ellps=intl
+towgs84=-86.277,-108.879,-120.181,0,0,0,0 +units=m +no_defs +type=crs".

Is there some way to force this transform through the operation context
methods/pipelines?

-- 
Peter Townsend
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