[PROJ] Combining custom transformations with EPSG

[Kristian Evers]

All,

I am working on moving the NKG transformations from the NKG-init file to
the PROJ database.
So far I have managed to re-create the transformation from ITRF2014 to the
Danish realization
of ETRS89. Broadly, I have done this by

1. setting up a new datum+CRS called NKG_ETRF00 (used as a hub for
   the various transformations defined by the NKG)
2. adding a concatenated operation from ITRF2014 to NKG_ETRF00, using
   three steps (ITRF2014 -> ITRF2000 -> ETRF2000 -> NKG_ETRF00)
3. adding another concatenated operation from NKG_ETRF00 to ETRS89(DK),
   this time using two steps (NKG_ETRF00 -> ETRF92 at 2000.0 -> ETRF92 at 1994.704
)

All of the above can be seen in this changeset:

https://github.com/kbevers/PROJ/commit/cabea1134d704c4e2cbf968442763565994f1c56

When querying the database with projinfo we can see that it works
as intended:

$ PROJ_LIB=./data/ ./src/projinfo -s EPSG:7789 -t EPSG:4936 --area denmark
-o PROJ
Candidate operations found: 2
-------------------------------------
Operation No. 1:

unknown id, Inverse of ITRF2000 to ITRF2014 (1) + ITRF2000 to ETRF2000 (2)
+ ETRF2000 to NKG_ETRF00 + NKG_ETRF00 to ETRF92 at 2000.0 + ETRF92 at 2000.0 to
ETRF92 at 1994.704, 1.11 m, Denmark - onshore and offshore.

PROJ string:
+proj=pipeline
 +step +inv +proj=helmert +x=-0.0007 +y=-0.0012 +z=0.0261 +rx=0 +ry=0 +rz=0
+s=-0.00212 +dx=-0.0001 +dy=-0.0001 +dz=0.0019 +drx=0 +dry=0 +drz=0
+ds=-0.00011 +t_epoch=2010 +convention=position_vector
 +step +proj=helmert +x=0.054 +y=0.051 +z=-0.048 +rx=0.000891 +ry=0.00539
+rz=-0.008712 +s=0 +dx=0 +dy=0 +dz=0 +drx=8.1e-05 +dry=0.00049
+drz=-0.000792 +ds=0 +t_epoch=2000 +convention=position_vector
 +step +proj=deformation +t_epoch=2000.0
+grids=eur_nkg_nkgrf03vel_realigned.tif
 +step +proj=helmert +x=0.03863 +y=0.147 +z=0.02776 +rx=0.00617753
+ry=5.064e-05
+rz=4.729e-05 +s=-0.00942 +convention=position_vector
 +step +proj=deformation +dt=-5.296 +grids=eur_nkg_nkgrf03vel_realigned.tif

-------------------------------------
Operation No. 2:

unknown id, Ballpark geocentric translation from ITRF2014 to ETRS89,
unknown accuracy, World, has ballpark transformation

PROJ string:
+proj=noop

Frankly, I am quite pleased about that. What I am not pleased about is that
I haven't
managed to integrate this completely with the rest of the database. As an
example, let's
say I want to transform between ITRF2008 and ETRS89(DK) instead:

$ PROJ_LIB=./data/ ./src/projinfo -s EPSG:5332 -t EPSG:4936 --area denmark
-o PROJ
Candidate operations found: 1
-------------------------------------
Operation No. 1:

unknown id, Ballpark geocentric translation from ITRF2008 to ETRS89,
unknown accuracy, World, has ballpark transformation

PROJ string:
+proj=noop

My expectation was that PROJ would connect the dots between my new
transformations and the
existing transformation between ITRF2008 and ITRF2014. PROJ of course knows
about it as
seen here:

$ PROJ_LIB=./data/ ./src/projinfo -s EPSG:5332 -t EPSG:7789 --area denmark
-o PROJ
Candidate operations found: 1
-------------------------------------
Operation No. 1:

EPSG:7790, ITRF2008 to ITRF2014 (1), 0.01 m, World.

PROJ string:
+proj=helmert +x=-0.0016 +y=-0.0019 +z=-0.0024 +rx=0 +ry=0 +rz=0 +s=2e-05
+dx=0
 +dy=0 +dz=0.0001 +drx=0 +dry=0 +drz=0 +ds=-3e-05 +t_epoch=2010
 +convention=position_vector


My question is, what have I missed? Should I add more records to the
database? Or is my
expectation of PROJ's abilities completely of the mark? I suspect the
problem might be that
what I want would require two hub-datums which I assume could be too big of
a combinatorial
task for PROJ to solve in a reasonable time and hence is not happening.

I am trying to avoid adding a bunch of concatenated operations from various
ITRFyyyy's
to NKG_ETRF00. Ideally I would skip the ITRF2014->ITF2000 step and let PROJ
handle that on
it's own. Is that even possible?

/Kristian
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