[PROJ] general use of proj_factors

[Javier Jimenez Shaw]

https://proj.org/en/9.5/development/reference/functions.html#c.proj_factors
You just provide the projected CRS, just created from the EPSG code. It
converts it internally into the projection over its geographic.

On Thu, 24 Oct 2024 at 15:19, Roger Oberholtzer <roger.oberholtzer at gmail.com>
wrote:

> On Thu, Oct 24, 2024 at 2:36 PM Javier Jimenez Shaw <j1 at jimenezshaw.com>
> wrote:
> >
> > The last versions also accept a projected CRS, extracting the projection
> from it.
>
> I am using 9.4.0. Do you mean that I should be able to use this?
>
> proj_normalize_for_visualization(0,
>                                        proj_create_crs_to_crs( 0,
>                                                                "EPSG:4326",
>                                                                "EPSG:3006",
>                                                                0))))
>
> If that's what you mean, it seems that the convergence values are not
> as expected. Or do you mean something else?
>
> >
> > On Thu, 24 Oct 2024, 14:27 Thomas Knudsen via PROJ, <
> proj at lists.osgeo.org> 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");
> >>
> >>
> >> Den tors. 24. okt. 2024 kl. 14.11 skrev Roger Oberholtzer via PROJ <
> proj at lists.osgeo.org>:
> >>>
> >>> 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
> >>> _______________________________________________
> >>> PROJ mailing list
> >>> PROJ at lists.osgeo.org
> >>> https://lists.osgeo.org/mailman/listinfo/proj
> >>
> >> _______________________________________________
> >> PROJ mailing list
> >> PROJ at lists.osgeo.org
> >> https://lists.osgeo.org/mailman/listinfo/proj
>
>
>
> --
> Roger Oberholtzer
>
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