[PROJ] general use of proj_factors

[Roger Oberholtzer]

Hi all,

I have been using proj_factors in a couple CRS to get the
median_convergence value. I need this when rotating a point that has
been projected into the CRS.

In my usual setup, I define something like the following (WGS84 (from
a GPS receiver) to Sweref99):

proj_normalize_for_visualization(0,
                                       proj_create_crs_to_crs( 0,
                                                               "EPSG:4326",
                                                               "EPSG:3006",
                                                               0))))
return(ISERROR);

This works fine. I get the points I expect. But as I will need to
rotate that point using data from the GPS (heading is true north), I
need to know how that differs from the projected point's north. So I
use proj_factors. As it turns out, I cannot use the definition from
above to do this. The median_convergence value is wrong. So what I
have been doing (for EPSG:3006 - it is CRS-specific) is defining
something like this:

proj_create(0, "+proj=utm +zone=33 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
+units=m +no_defs");

You will note that this is the implementation of EPSG:3006, but
without +type=crs. Why without that parameter? Time. When including
it, the proj_factors call takes around 60 times longer. This is
extremely significant when generating, say, point clouds with hundreds
of million points. A processing time of 15 minutes becomes 15 hours.
The values provided are exactly the same. But one takes significantly
longer to provide them.

I see this effect in more than EPSG:3006. For example EPSG:5677. I
suspect that it is generally the case.

What I would like to do is something like this:

proj_normalize_for_visualization(0,
                                       proj_create_crs_to_crs( 0,
                                                               "EPSG:4326",
                                                               "EPSG:3006",
                                                               0))))
return(ISERROR);

proj_create(0, "EPSG:3006");


Where the first is used to project the point, and the second is used
to call proj_factors for that point.

This would allow a general solution, which is what I am after. So I
either need to call something other than proj_create, or I need to see
how to remove +type=crs in this context.

Does this make sense?

-- 
Roger Oberholtzer