How to project a point from from one ellipsoid to other ellipsoid using proj library

Hi all,
I am writing to seek guidance on how to project a point from one ellipsoid to another ellipsoid using the Proj library.
I have been working on a geospatial project that requires converting coordinates between different ellipsoids. After some research, I discovered that the Proj library is a widely used and powerful tool for geospatial data transformations. However, I am currently facing some challenges in understanding and implementing the ellipsoid projection functionality.
I would greatly appreciate it if you could provide me with some guidance on how to accomplish this task using the Proj library. Specifically, I would like to know:

  1. I want to project a 3d point which is in wgs84 to an airy ellipsoid.
    I did as shown below. But I am getting incorrect values and height of the point is also not changing.

PJ *Src = proj_create(C, “+proj=geocent +datum=WGS84 +type=crs”);
PJ *Targ_Coor = proj_create(C, “+proj=geocent +ellps=airy +type=crs”);
auto P2G = proj_create_crs_to_crs_from_pj(C, Src, Targ_Coor, NULL, NULL);
if (!P2G)
return result;

c_out_glob = proj_trans(P2G, PJ_FWD, c_in);

  1. Are there any specific parameters or options that I need to consider while performing the projection from 3d to 2d? In the below code, I am performing projection of a point from wgs84 to oblique merctor. But still I am getting incorrect values.

PJ *Src = proj_create(C, “+proj=geocent +datum=WGS84 +type=crs”);
if (Src == NULL)
return result;

   PJ *Targ = proj_create(C, "+proj=omerc +lat_0=4 +lonc =115 +k_0=1 +x_0=2000000 +y_0=5000000 +lat_ts=0 +h_0=0 +alpha=53.31581 +gamma=53.130102  +lat_ts=0 +units=m +ellps=WGS84 +type=crs");

   if (Targ == NULL)
         return result;

   auto G2P = proj_create_crs_to_crs_from_pj(C, Src, Targ, NULL, NULL);
   if (!G2P)
         fprintf(stderr, "failed \n");
         return result;
   PJ *norm = proj_normalize_for_visualization(C, G2P);
   if (0 == norm) {
         fprintf(stderr, "Failed to normalize transformation object.\n");
         return 1;

   G2P = norm;

c_out = proj_trans(G2P, PJ_FWD, c_in);
Any code examples, documentation references, or step-by-step instructions you could provide would be immensely helpful in guiding me through this process. I am eager to learn and explore the capabilities of the Proj library, and any assistance you can offer would be invaluable.
Thank you very much for your time and support.

This is the GMT forum; try a PROJ forum.