[PROJ] general use of proj_factors

[Roger Oberholtzer]

On Thu, Oct 24, 2024 at 2:27 PM Thomas Knudsen <knudsen.thomas at gmail.com> wrote:
>
> Roger,
>
> The "factors" elements refer to a projection, not to a CRS
> (the CRS includes reference frame information), and not
> to a transformation (which describes a path between two
> different CRS).
>
> So you should only call proj_factors on a PJ object obtained
> from a plain classic proj(ection) string, i.e:
>
> p = proj_create(0, "proj=utm  zone=33  ellps=GRS80");

I'm okay with that concept. I am just trying to make this more general
in usage. So if a user specifies something like EPSG:5678, I would
have to know that the corresponding spec for proj_create is:

+proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel
+towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +units=m +no_defs

Which is  the same as projinfo lists for EPSG:5678, but without the
+type=crs. Also note that the +towgs84 seems not to effect anything.
The convergence is the same with or without it, and the processing
time is the same.

Is it just to obtain this string from proj as projinfo does, and
remove unwanted items like +type=crs? Or have I missed some great
thing in proj that addresses this?

-- 
Roger Oberholtzer