regression: projections need to be a crs

[josch]

Hi,

thanks a lot for implementing proj7 support in 88241b3. Unfortunately, that commit introduces a call to proj_create_crs_to_crs which means that projections must now be a crs. I was using this pipeline before:

+proj=pipeline +step +proj=webmerc +step +proj=tinshift +file=/tmp/tinshift.json

and that doesn't work anymore as proj_create_crs_to_crs will fail with

proj_create_operations: source_crs is not a CRS

(It took me a while to figure out where the error comes from, so I also filed #4305 to improve error reporting)

Is this change intentional and do projections need to be a crs from now on? Projections that were not a crs worked fine in the past.

[artemp]

@josch 👍 thanks for highlighting^! According to proj docs cs2cs is using proj_create_crs_to_crs also. Is it possible to modify pipeline init string you mentioned above to work with cs2cs (proj9) ?

[josch]

According to OSGeo/PROJ#3161 (comment) that string cannot be turned into a crs, so no.

[josch]

Hi @artemp, I wanted to ask what your plans are. Do you plan to add another codepath that also supports projections that are not a crs? Or do you plan to drop support for non-crs projections with the next release? Thanks!

[mathisloge]

Just to note: with #4305 minimum proj version is bumpt up to 8.0.0 from 7.2.0

proj_context_errno_string was added with proj 8.0.0 https://proj.org/development/reference/functions.html#c.proj_context_errno_string

@artemp this was detected with the docker image PR #4267 since it is using ubuntu 21.04. The new ubuntu LTS 22.04 has currently 8.2.1 (https://packages.ubuntu.com/search?suite=default&section=all&arch=any&keywords=libproj-dev&searchon=names)

[artemp]

Just to note: with #4305 minimum proj version is bumpt up to 8.0.0 from 7.2.0

proj_context_errno_string was added with proj 8.0.0 https://proj.org/development/reference/functions.html#c.proj_context_errno_string

@artemp this was detected with the docker image PR #4267 since it is using ubuntu 21.04. The new ubuntu LTS 22.04 has currently 8.2.1 (https://packages.ubuntu.com/search?suite=default&section=all&arch=any&keywords=libproj-dev&searchon=names)

@mathisloge I think we should use proj_context_errno_string conditionally based on PROJ_VERSION

[artemp]

Hi @artemp, I wanted to ask what your plans are. Do you plan to add another codepath that also supports projections that are not a crs? Or do you plan to drop support for non-crs projections with the next release?

@josch I'm trying to figure out if this is something we can support. It would be super useful to see c++ code using proj_create to achieve this non-crs pipeline Do you have any pointers?

[josch]

@mathisloge I think we should use proj_context_errno_string conditionally based on PROJ_VERSION

I filed #4307 to fix this.

@josch I'm trying to figure out if this is something we can support. It would be super useful to see c++ code using proj_create to achieve this non-crs pipeline Do you have any pointers?

I'm afraid I'm a complete proj noob. That's why I asked how to do it in OSGeo/PROJ#3161 (comment) but as the issue is closed I don't expect a reply.

I also asked the more general question on stackoverflow https://stackoverflow.com/questions/71823105 because all code I've seen so far that migrates to more recent proj versions just uses proj_create_crs_to_crs and calls it a day.

Since this is breaking my use-case for mapnik I'm willing to submit a pull request with an alternative code-path for non-crs input projections but first I have to learn how to do it in the first place. :(