[PROJ] Always using tinshift when dealing with KKJ

[Oskari Timperi]

Hi,

I was testing out the tinshift for transforming KKJ (EPSG:2393 etc) to ETRS89 / TM35FIN(E,N) (EPSG:3067). This seems to work great!

But, now if I want to transform from let's say EPSG:2393 to EPSG:4326, proj doesn't use the tinshift. Here's output from projinfo:

~> projinfo -s EPSG:2393 -t EPSG:4326 -o proj
Candidate operations found: 1
-------------------------------------
Operation No. 1:

unknown id, Inverse of Finland Uniform Coordinate System + KKJ to WGS 84 (2), 1 m, Finland - onshore.

PROJ string:
+proj=pipeline
  +step +proj=axisswap +order=2,1
  +step +inv +proj=tmerc +lat_0=0 +lon_0=27 +k=1 +x_0=3500000 +y_0=0 +ellps=intl
  +step +proj=push +v_3
  +step +proj=cart +ellps=intl
  +step +proj=helmert +x=-96.062 +y=-82.428 +z=-121.753 +rx=-4.801 +ry=-0.345
        +rz=1.376 +s=1.496 +convention=coordinate_frame
  +step +inv +proj=cart +ellps=WGS84
  +step +proj=pop +v_3
  +step +proj=unitconvert +xy_in=rad +xy_out=deg
  +step +proj=axisswap +order=2,1

Is it possible to make proj use the tinshift for this particular transformation? How about using the tinshift generally when dealing with KKJ, basically always pivoting through EPSG:3067 regardless of what the other system was?

I read the projinfo documentation and noticed information about auxiliary databases, would this be what I'm after?

Is there any documentation (or tutorials) of what kind of changes I could possibly do with auxiliary databases?

If I'm understanding correctly and the auxiliary database is what I need, do you have any tips how I would solve this issue?

I'm currently using PROJ 7.2.1, but I tested also with 8.1.0 (the output above is from the newer version).

Thanks,

- Oskari