Pipeline doesn't honour +datum=NAD27 expansion to +ellps=clrk66 +nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat

[rouault]

The following results in a unexpected no-op

$ echo "-100 40 0" | PROJ_LIB=nad src/cct +proj=pipeline +step +proj=longlat +datum=NAD27 +inv +step +proj=longlat +datum=WGS84
-100.0000000000   40.0000000000        0.0000           inf

whereas

$ echo "-100 40 0" | PROJ_LIB=nad src/cct +proj=pipeline +step +proj=longlat +ellps=clrk66 +nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat +inv +step +proj=longlat +datum=WGS84
-100.0004058367   40.0000058947        0.0000           inf

which is the same as

$ echo "-100 40 0" | PROJ_LIB=nad src/cs2cs +proj=longlat +datum=NAD27 +to +proj=longlat +datum=WGS84
100d0'1.461"W	40d0'0.021"N 0.000

[kbevers]

I'll just re-iterate Thomas' explanation of what's going on here: http://lists.maptools.org/pipermail/proj/2018-February/008055.html

The problem can possibly be fixed by expanding +datum=xxx to it's full representation from pj_datums in the top of cs2cs_emulation_setup() and adding the expanded parameters to P->params before the actual setup is started. Exercise left to the reader :-)

[kbevers]

Okay, I think I've got a fix for this. It seems to be straight forward actually. Pull request coming up.