Re: [Qgis-user] No transform available between XXX and EPSG:4326 when using non-Earth projections

[Even Rouault]

Le 18/03/2021 à 23:25, Nyall Dawson a écrit :
On Fri, 19 Mar 2021 at 07:04, Even Rouault <even.roua...@spatialys.com> wrote:
Sebastian,
I can reproduce the 2 error messages you got. There's indeed something in QGIS 
that tries a terrestrial reprojection and should be fixed, but I don't think 
this is really critical apart of the annoyance of being spammed about that. 
Please file a QGIS issue to track this.
There's two parts in particular which cause this warning to be shown,
and indeed make an assumption that lat/lng === EPSG:4326
- The status bar coordinate widget
- The CRS selector "area of use" preview widget

We probably need api for testing "is non earth crs" and "given a
non-earth crs, give me a lat/long crs for the same planetary body" to
resolve these. But as Even has pointed out neither are critical
functionality, and the warning can be ignored.

At the PROJ API level, you can extract the underlying geodetic CRS 
(which in 99% of the cases will be a geographic one) of a CRS (be it a 
geographic, projected or compound one) with proj_crs_get_geodetic_crs(). 
proj_normalize_for_visualization() might be needed to get a consistent 
lon, lat axis order.

The "advantage" of this is that if you just want to see the lat, long of 
any projected CRS without requiring the datum to be WGS 84, it will work 
and without the complications of datum adjustments. And you don't need 
to distinguish Earth vs non-Earth use cases.

Pedantic note: if the CRS is a Geocentric CRS, then 
proj_crs_get_geodetic_crs() will return it, and not a geographic CRS as 
one might perhaps expect. A geodetic CRS may be a geocentric (X,Y,Z) or 
a geographic (lat,lon lon,lat) one). To get a geographic CRS from that, 
you'd need to extract its datum ( proj_crs_get_datum() and then build a 
geographic CRS with proj_create_geographic_crs_from_datum() ), but 
that's quite unlikely to be needed in QGIS use cases.


Nyall


That said, I managed to do a non-Earth reprojection by adding a custom CRS with 
the following WKT definition:

GEOGCRS["MARS Geographic",

DATUM["D_MARS",

ELLIPSOID["MARS",3396000,0,

LENGTHUNIT["metre",1,

ID["EPSG",9001]]]],

PRIMEM["Reference meridian",0,

ANGLEUNIT["degree",0.0174532925199433,

ID["EPSG",9122]]],

CS[ellipsoidal,2],

AXIS["longitude",east,

ORDER[1],

ANGLEUNIT["degree",0.0174532925199433,

ID["EPSG",9122]]],

AXIS["latitude",north,

ORDER[2],

ANGLEUNIT["degree",0.0174532925199433,

ID["EPSG",9122]]]]

and selecting it as the project CRS. Using ESRI:104905 (GCS_Mars_2000) seems to 
also work (I haven't deeply checked but I do get a reprojected raster and 
geographic coordinates), but you'll get each time the warnings about the 2 
terrestrial coordinate operations. Hopefully in coming months, I'll be able to 
ingest the IAU catalog into the PROJ database as well.

Even

Le 18/03/2021 à 21:07, Sebastian Walter a écrit :

Hi,

Since Version 3.X I'm not able to perform transformations on planetary 
(non-Earth) data (from e.g. Mars or Mercury, example dataset: 
https://maps.planet.fu-berlin.de/mex4/h1104_0000.ihs.53.tif). The error message 
is:

No transform is available between SINUSOIDAL MARS and Custom CRS: 
GEOGCRS["unknown",DATUM["unknown",ELLIPSOID["WGS 8….
proj_create_operations: Source and target ellipsoid do not belong to the same 
celestial body
No transform available between SINUSOIDAL MARS and EPSG:4326

The consequence is that transformations to lat/lon are not possible any more. With the 
older QGis there was no problem like this. My suspect is the "new" Proj6 
database layout, but the above error messages also point to the fact there there is some 
EPSG:4326 projection hard-coded into Qgis.

Is anybody successfully using Qgis with non-Earth data and proper 
transformations?

Thanks for any advise...

Sebastian



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