[PROJ] Calculating meridian convergence

[Even Rouault]

Roger,

I believe the issue here is that you use a transformation pipeline that 
expects input coordinates to be in degrees.

You should just use your code with

PJ* proj = proj_create(0, "EPSG:3006");

Even

Le 01/06/2022 à 11:49, Roger Oberholtzer a écrit :
> 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.
>
-- 
http://www.spatialys.com
My software is free, but my time generally not.