[PROJ] Explaining geodesic distance discrepancies

[Alan Snow]

This may help getting the factors:
https://pyproj4.github.io/pyproj/stable/api/proj.html#pyproj.proj.Proj.get_factors

On Fri, May 15, 2020, 5:33 PM Charles Karney <charles at karney.com> wrote:

> I believe PROJ does have some way of computing the scale of a
> projection; but it uses numerical differencing.  GeographicLib computes
> the scale of TM from an analytic expression, so it's probably more
> accurate.  The command I used to get the scale (using GeographicLib) is:
>
>      echo 0.5 |
>      GeodSolve -I 31.4617 -83.4849 31.4704 -83.4781 -F -p 10 |
>      cut -f1,2 -d' ' | GeoConvert -c -p 10
> =>
>      -1.2959245331415 1.000285683174366
>
> The GeodSolve expression gets the midpoint of a geodesic.  GeoConvert -c
> returns the meridian convergence and scale for the UTM projection.
>
> The command line utility geod uses the same code as pyproj.Geod.  So
> unless you have extra versions of geod lying around they should return
> the same results.
>
>
> On 5/15/20 5:00 PM, Dima Kogan wrote:
> > Thank you very much. That's VERY useful info. How did you get that scale
> > factor? Is there a proj tool and/or function call?
> >
> > I have just tried to run the "geod" cmd on another box with another proj
> > install, and it matches up over there. Which is probably enough for me
> > to go figure this out. We DO expect the "geod" cmdline and the
> > pyproj.Geod results to match 100%, right?
> >
> > Thanks again.
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