[PROJ] Proj 6 API questions

[Nyall Dawson]

On Mon, 18 Mar 2019 at 17:34, Kristian Evers <kreve at sdfe.dk> wrote:
>
> I may be wrong but I don’t think there are acronyms for them. It’s based
> on the EPSG dataset and acronyms are not assigned in that regime. Do
> you actually need them or is this just so that it fits better in existing QGIS
> infrastructure?

There's two things QGIS uses this for:

1. When a user selects the CRS for their project, the project
ellipsoid (used when calculating geodetic distances/areas for the
project) is automatically set to match the ellipsoid corresponding to
this CRS. i.e. select EPSG:28356 and the project ellipsoid is set to
GRS80, set it to EPSG:102013 and you get International 1909.  So we
need a way to get the ellipsoid from an acronym (or alternatively,
directly from a CRS).

2. When performing geodetic distance/area calculations, QGIS
transforms coordinates from the layer's source CRS to a crs crafted
from "+proj=longlat +ellps=XXXXX +nodefs", and then uses the result
when performing the geodetic calculations. So we need a way to obtain
the ellipsoid acronym given an ellipsoid definition (where a
corresponding one exists).

(Full disclaimer: I'm not fully aware of the background behind this
logic, and there may be superior logic to follow. Let me know if you
think there is!)

Nyall


>
> We could potentially assign acronyms for all ellipsoids but I fear that will
> become a maintenance mess whenever the EPSG dataset is updated.
>
> /Kristian
>
> > On 18 Mar 2019, at 08:27, Nyall Dawson <nyall.dawson at gmail.com> wrote:
> >
> > On Mon, 18 Mar 2019 at 16:56, Kristian Evers <kreve at sdfe.dk> wrote:
> >>
> >> You use proj_get_authorities_from_database() for that. Generally there’s functions
> >> for retrieving all info like that, just search around in the function reference a bit and you’ll
> >> find it.
> >
> > Ok, that part is working, and I'm seeing a LOT more! But now I'm a bit
> > confused... if I use this approach to iterate through the ellipsoids,
> > is there anyway to get the ellipsoid acronym from them? Using
> > proj_list_ellps I could get PJ_ELLPS->id.
> >
> > Nyall
> >
> >
> >
> >>
> >> /Kristian
> >>
> >> On 18 Mar 2019, at 07:52, Nyall Dawson <nyall.dawson at gmail.com> wrote:
> >>
> >> On Mon, 18 Mar 2019 at 16:42, Kristian Evers <kreve at sdfe.dk> wrote:
> >>
> >>
> >> Hi Nyall,
> >>
> >> I am very happy to see that you have started working on this!
> >>
> >> You will probably get less discrepancies between the QGIS list of ellipsoids
> >> and the PROJ equivalent if you instead use proj_get_codes_from_database
> >> using type PJ_TYPE_ELLIPSOID.
> >>
> >>
> >> What should I use as auth_name if I want to get ALL ellipsoids? The
> >> dox state it must not be null...
> >>
> >> Thanks for the detailed pointers, I'll let you know the result!
> >>
> >> Nyall
> >>
> >>
> >> You should then be able to get the
> >> parameters with proj_ellipsoid_get_parameters. This is untested on my part
> >> but I believe that is the general workflow you have to go through to get the
> >> ellipsoid out of proj.db.
> >>
> >> proj_list_ellps() simply shows you the same list as you would get from calling
> >> `proj -le`, i.e. ellipsoids that can be used with `+ellps=xxx` parameters. This
> >> is confusing and I shall think of a way to change the docs so it is less confusing :-)
> >>
> >> It would be interesting to see the difference between the ellipsoid tables from
> >> The QGIS and PROJ databases.
> >>
> >> /Kristian
> >>
> >> On 18 Mar 2019, at 04:39, Nyall Dawson <nyall.dawson at gmail.com> wrote:
> >>
> >> Hi list,
> >>
> >> So (thanks for sponsorship from the ICSM), I'm working on porting QGIS
> >> to the Proj 6 API. I suspect I'll have many more questions (especially
> >> as I attempt to drop QGIS' own custom projection database in favour of
> >> the upstream proj one). Let's get started...
> >>
> >> I thought I'd begin with a simple task of replacing QGIS' ellipsoid db
> >> with proj's, using proj_list_ellps(). QGIS has this table of
> >> "standard" ellipsoids, built up from... somewhere... in the vast
> >> history of the QGIS project. When comparing against proj's db, it
> >> includes many items which are missing in proj's:
> >>
> >> Adrastea2000: Adrastea2000 10000.0/7000.0
> >> Amalthea2000: Amalthea2000 125000.0/64000.0
> >> Ananke2000: Ananke2000 10000.0/10000.0
> >> Ariel2000: Ariel2000 581100.0/577700.0
> >> Atlas2000: Atlas2000 18500.0/13500.0
> >> Belinda2000: Belinda2000 33000.0/33000.0
> >> Bianca2000: Bianca2000 21000.0/21000.0
> >> Callisto2000: Callisto2000 2409400.0/2409300.0
> >> Calypso2000: Calypso2000 15000.0/8000.0
> >> Carme2000: Carme2000 15000.0/15000.0
> >> Charon2000: Charon2000 593000.0/593000.0
> >> IGNF:ELG004: Clarke 1866 6378206.4/6356583.799998981
> >> IGNF:ELG003: Clarke 1880 Anglais 6378249.1453/6356514.8698
> >> IGNF:ELG010: Clarke 1880 IGN 6378249.2/6356514.999978254
> >> cape: Clarke 1880 mod. 6378249.145/6356514.966398753
> >> Cordelia2000: Cordelia2000 13000.0/13000.0
> >> Cressida2000: Cressida2000 31000.0/31000.0
> >> Deimos2000: Deimos2000 7500.0/5200.0
> >> Desdemona2000: Desdemona2000 27000.0/27000.0
> >> Despina2000: Despina2000 74000.0/74000.0
> >> Dione2000: Dione2000 560000.0/560000.0
> >> Earth2000: Earth2000 6378140.0/6356750.0
> >> Elara2000: Elara2000 40000.0/40000.0
> >> Enceladus2000: Enceladus2000 256300.0/244600.0
> >> Epimetheus2000: Epimetheus2000 69000.0/55000.0
> >> Europa2000: Europa2000 1564130.0/1560930.0
> >> Galatea2000: Galatea2000 79000.0/79000.0
> >> Ganymede2000: Ganymede2000 2632400.0/2632350.0
> >> Helene2000: Helene2000 16000.0/16000.0
> >> Himalia2000: Himalia2000 85000.0/85000.0
> >> Hyperion2000: Hyperion2000 164000.0/107000.0
> >> IGNF:ELG108: IAG GRS 1967 6378160.0/6356774.5161
> >> IGNF:ELG037: IAG GRS 1980 6378137.0/6356752.314140356
> >> Iapetus2000: Iapetus2000 718000.0/718000.0
> >> IGNF:ELG001: International-Hayford 1909 6378388.0/6356911.9461279465
> >> Io2000: Io2000 1829400.0/1815700.0
> >> Janus2000: Janus2000 97000.0/77000.0
> >> Juliet2000: Juliet2000 42000.0/42000.0
> >> Jupiter2000: Jupiter2000 71492000.0/66854000.0
> >> Larissa2000: Larissa2000 96000.0/96000.0
> >> Leda2000: Leda2000 5000.0/5000.0
> >> Lysithea2000: Lysithea2000 12000.0/12000.0
> >> Mars2000: Mars2000 3396190.0/3376200.0
> >> Mercury2000: Mercury2000 2439700.0/2439700.0
> >> Metis2000: Metis2000 21500.0/21500.0
> >> Mimas2000: Mimas2000 209100.0/191400.0
> >> Miranda2000: Miranda2000 240400.0/232900.0
> >> Moon2000: Moon2000 1737400.0/1737400.0
> >> Naiad2000: Naiad2000 29000.0/29000.0
> >> Neptune2000: Neptune2000 24764000.0/24341000.0
> >> Nereid2000: Nereid2000 170000.0/170000.0
> >> Oberon2000: Oberon2000 761400.0/761400.0
> >> Ophelia2000: Ophelia2000 15000.0/15000.0
> >> IGNF:ELG017: PLESSIS 1817 6376523.0/6355862.933255573
> >> Pan2000: Pan2000 10000.0/10000.0
> >> Pandora2000: Pandora2000 55000.0/31000.0
> >> Pasiphae2000: Pasiphae2000 18000.0/18000.0
> >> Phobos2000: Phobos2000 13400.0/9200.0
> >> Phoebe2000: Phoebe2000 115000.0/105000.0
> >> Pluto2000: Pluto2000 1195000.0/1195000.0
> >> Portia2000: Portia2000 54000.0/54000.0
> >> Prometheus2000: Prometheus2000 74000.0/34000.0
> >> Proteus2000: Proteus2000 218000.0/201000.0
> >> Puck2000: Puck2000 77000.0/77000.0
> >> Rhea2000: Rhea2000 764000.0/764000.0
> >> Rosalind2000: Rosalind2000 27000.0/27000.0
> >> IGNF:ELG032: SPHERE PICARD 6371598.0/6371598.0
> >> Saturn2000: Saturn2000 60268000.0/54364000.0
> >> Sinope2000: Sinope2000 14000.0/14000.0
> >> Telesto2000: Telesto2000 15000.0/7500.0
> >> Tethys2000: Tethys2000 535600.0/525800.0
> >> Thalassa2000: Thalassa2000 40000.0/40000.0
> >> Thebe2000: Thebe2000 58000.0/42000.0
> >> Titan2000: Titan2000 2575000.0/2575000.0
> >> Titania2000: Titania2000 788900.0/788900.0
> >> Triton2000: Triton2000 1352600.0/1352600.0
> >> Umbriel2000: Umbriel2000 584700.0/584700.0
> >> Uranus2000: Uranus2000 25559000.0/24973000.0
> >> Venus2000: Venus2000 6051800.0/6051800.0
> >> IGNF:ELG052: WGS72 6378135.0/6356750.520016094
> >> IGNF:ELG102: WGS72 (NWL-10F) 6378135.0/6356750.520016094
> >> IGNF:ELG053: WGS84 6378137.0/6356752.314245179
> >>
> >> The vast majority of these are planetary ellipsoids/moons, but there's
> >> others which are less obvious (cape/IGNF*). So
> >>
> >> Question 1: should the non-Earth ellipsoids be included in the proj database?
> >>
> >> Question 2: What about cape/IGNF* ? Should they be included in proj,
> >> or are they mistakes and should not even be included in the current
> >> QGIS db?
> >>
> >> Nyall
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> >>
> >>
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>