unwanted use of WGS84 ellipsoid parameters when using Mars ellipsoid parameters

[rouault]

Originally reported as OSGeo/gdal#665 , but can be reproduced with proj

$ echo "0 0 0" | src/cs2cs +proj=longlat +a=3396190 +b=3376200 +no_defs +to +proj=geocent +a=3396190 +b=3376200 +lon_0=0 +units=m +no_defs
3396190.00	0.00 0.00

which is expected. But doing the same with cct leads to

$ echo "0 0 0" | src/cct +proj=pipeline +step +proj=longlat +a=3396190 +b=3376200 +inv +step +proj=geocent +a=3396190 +b=3376200 +lon_0=0 +units=m
 6378137.0000         0.0000        0.0000           inf

We somehow use WGS84

Same with simplified

$ echo "0 0 0" | src/cct  +proj=geocent +a=3396190 +b=3376200 +lon_0=0 +units=m
 6378137.0000         0.0000        0.0000           inf

[kbevers]

I haven't looked at the code yet but my spidey sense is tingling at the use of geocent with cct. geocent has special logic that only works in pj_transform() as far as I remember. It should be removed alongside proj_api.h.

For this sort of thing the correct operation to use is cart which works as intended:

echo "0 0 0" | cct  +proj=cart +a=3396190 +b=3376200 +lon_0=0 +units=m
 3396190.0000         0.0000        0.0000           inf

This needs to be documented.