seeking advice for proj_create_crs_to_crs and tinshift transform

[josch]

Dear Proj maintainers,

thanks a lot for your amazing help with implementing the fallback_strategy option for tinshift in #2903 and #2907. I'm using this to warp maps and render them with mapnik. This all worked well until mapnik added support for more recent proj >= 7 and with it added a call to proj_create_crs_to_crs: mapnik/mapnik@88241b3#diff-3817884b302c98d6b02df071fa796efda25b53389fcedf28f6b7c200a7ed5efdR130

My tinshift pipeline +proj=pipeline +step +proj=webmerc +step +proj=tinshift +file=/tmp/tinshift.json is not a crs so with mapnik from git, my code will fail with proj_create_operations: source_crs is not a CRS.

Unfortunately I'm still a proj noob and while I found +type=crs in the docs, adding it to my pipeline didn't solve the problem.

So my question to you is: is there a way to turn my proj string into a crs? And if not, what should mapnik replace proj_create_crs_to_crs with so that it still works with a non-crs source?

Thank you!

[rouault]

So my question to you is: is there a way to turn my proj string into a crs?

no

And if not, what should mapnik replace proj_create_crs_to_crs with so that it still works with a non-crs source?

it could offer the possibility to handle a PROJ transformation pipeline by using proj_create

[josch]

Thanks! Do you know of existing code using proj_create to allow non-crs input? When searching for code examples it seems that the world moved to proj_create_crs_to_crs. :(

Do I also understand it correctly, that to get a single transformation for use with proj_trans from source to target with my source containing a pipeline would result in a nested pipeline?

[snowman2]

Do you know of existing code using proj_create to allow non-crs input?

pyproj may be a useful reference:

• proj_create: Transformer.from_pipeline
• proj_create_crs_to_crs: Transformer.from_crs

[josch]

Than you @snowman2, I asked the same question on the ML and got a very helpful reply from @rouault: https://lists.osgeo.org/pipermail/proj/2022-April/010597.html

I'll reply to that mail and update my question on stackoverflow once I have some code. :)