[PROJ] Meridian convergence

[Kristian Evers]

Roger,

I believe using proj_factors() is the fastest way, yes. The current implementation has its roots in the very early days of PROJ.
If we were to implement the same functionality from scratch today they would likely be split into more smaller functions with
one purpose each. You could always implement your own custom version of it based on the PROJ code. Start here: 
https://github.com/OSGeo/PROJ/blob/master/src/factors.cpp

I doubt that your code is particularly useful, to be honest. If your are indeed using EPSG:4326 I believe you will get a meridian
convergence of 0.0 no matter what the input. The reason being that EPSG:4326 is not a projected CRS and hence you haven’t
got a grid north that can deviate from true north. Also, EPSG:4326 is not SWEREF99 - it is the WGS84 datum ensemble given
in geodetic coordinates. You are probably looking for one of the SWEREF99 TM CRS’s.

The meridian convergence calculation should work for most, if not any, projected CRS.

/Kristian

> On 25 Mar 2024, at 08.44, Roger Oberholtzer via PROJ <proj at lists.osgeo.org> wrote:
> 
> I have been using proj_factors() to get the meridian convergence for a given point. It has been working great. We need this when projecting LiDAR points that involve multiple rotations. The code is something like this:
> 
>        PJ_FACTORS factors; 
>        PJ_COORD vals;
> 
>        vals.lp.lam = DegreesToRadians(point->LONGITUDE); 
>        vals.lp.phi = DegreesToRadians(point->LATITUDE); 
>         
>        factors = proj_factors(tinfo->Meridian, vals); 
> 
>        return(RadiansToDegrees(factors.meridian_convergence));
> 
> I have been using it with Sweref99 (EPSG:4326).
> 
> I have two questions:
> 
> Is proj_factors() the fastest way to get this information? A look at the code shows that it is calculating other things as well. Is there is a faster way? When generating point clouds, each contains 10s of millions of points, and we generated 10s of thousands of these over a summer. Needless to say, we are always looking for speedups.
> 
> Would this method work for all 'standard' projections? By 'standard' I mean the typical ones used in, say, European countries. Would we need to consider anything when extending this to something other than EPSG:4326?
> 
> --
> Roger Oberholtzer
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