[PROJ] Calculating meridian convergence

[Roger Oberholtzer]

I am trying to determine the meridian convergence for a point. The
projection is defined as:

PJ *proj = proj_normalize_for_visualization(0,
           proj_create_crs_to_crs(0, "EPSG:4326", "EPSG:3006", 0));

The source lat/longs are wgs84 from a GPS receiver. The projection is
Sweref99. That is working just fine. In my case, the LONG/LAT are in
degrees.

       PJ_COORD ivals, vals;
       vals.xyz.x = LONG;
       vals.xyz.y = LAT;
       vals.xyz.z = HEIGHT;
       ivals = proj_trans(tinfo->Proj, PJ_FWD, vals);

So far, so good.

I need to do some rotations of the projected points. Towards that end,
I would like to determine the meridian convergence. I am expecting to
use this as a correction to a yaw value that my GPS receiver also
provides via an integrated IMU.

       PJ_FACTORS factors;

       vals.lp.lam = proj_torad(LONG);
       vals.lp.phi = proj_torad(LAT);

       factors = proj_factors(tinfo->Proj, vals);

I must be doing something incorrectly, as I get similar results for
data from different locations. Or I am missing a step. Or I am just
doing something totally wrong.

Data from west of the Sweref99 central meridian (convergence should be
-1.56713° according to the Swedish authority):

LAT 55.600783
LONG 13.100962
ALT 13.463000
E 380351.081178
N 6163285.609844
angular_distortion 0.567652
meridian_parallel_angle 1.570796
meridian_convergence -0.004468

Data from east of the Sweref99 central meridian (convergence should be
0.40007° according to the Swedish authority):

LAT 58.618118
LONG 15.468616
ALT 79.686000
E 527220.410693
N 6497625.569591
angular_distortion 0.645715
meridian_parallel_angle 1.570796
meridian_convergence -0.004696

angular_distortion, meridian_parallel_angle, and meridian_convergence
are the values returned by proj_factors().

This is with proj 8.2.1 on Linux. It is generally working fine. It's just this.

Any pointers would be greatly appreciated.

-- 
Roger Oberholtzer