[PROJ] Units in a derived system

[Javier Jimenez Shaw]

Hi

I am trying to make a derived projected CRS... in feet.
The conversion method is a "PROJ-based operation method" with an affine
transformation (simplified here with just xoff of 20 feet).
https://proj.org/operations/transformations/affine.html

The derived system is something like this (my data is 3D, but in 2D is also
reproduced):

$ cat derived.wkt

DERIVEDPROJCRS["Custom Site Calibrated CRS",
    BASEPROJCRS["NAD83(2011) / Mississippi East (ftUS)",
        BASEGEOGCRS["NAD83(2011)",
            DATUM["NAD83 (National Spatial Reference System 2011)",
                ELLIPSOID["GRS 1980",6378137,298.257222101,
                    LENGTHUNIT["metre",1]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433]]],
        CONVERSION["SPCS83 Mississippi East zone (US Survey feet)",
            METHOD["Transverse Mercator",
                ID["EPSG",9807]],
            PARAMETER["Latitude of natural origin",29.5,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8801]],
            PARAMETER["Longitude of natural origin",-88.8333333333333,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8802]],
            PARAMETER["Scale factor at natural origin",0.99995,
                SCALEUNIT["unity",1],
                ID["EPSG",8805]],
            PARAMETER["False easting",984250,
                LENGTHUNIT["US survey foot",0.304800609601219],
                ID["EPSG",8806]],
            PARAMETER["False northing",0,
                LENGTHUNIT["US survey foot",0.304800609601219],
                ID["EPSG",8807]]]],
    DERIVINGCONVERSION["Affine transformation as PROJ-based",
        METHOD["PROJ-based operation method: +proj=pipeline +step
+proj=affine +xoff=20"]],
    CS[Cartesian,2],
        AXIS["easting (X)",east,
            ORDER[1],
            LENGTHUNIT["US survey foot",0.304800609601219]],
        AXIS["northing (Y)",north,
            ORDER[2],
            LENGTHUNIT["US survey foot",0.304800609601219]],
    REMARK["EPSG:6507 with 20 feet offset"]]

But the output I get seems to be in meters (I deduced that running with
PROJ_DEBUG=3) It is kind of ignoring the CS.
$ echo 29 -88 0 | cs2cs EPSG:6318 EPSG:6507
1250642.38 -180875.29 0.00
$ echo 29 -88 0 | cs2cs EPSG:6318 "$(cat derived.wkt)"
381216.56 -55130.90 0.00

If I concatenate the pipeline with cct, I get the result I was expecting
(20 feet more in the x):
$ echo 29 -88 0 | cs2cs EPSG:6318 EPSG:6507 | cct +proj=pipeline +step
+proj=affine +xoff=20
1250662.3800   -180875.2900        0.0000

Is that expected? Am I doing anything wrong?

Thanks.
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
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