From charles.karney at gmail.com  Wed Oct  1 06:53:03 2025
From: charles.karney at gmail.com (Charles Karney)
Date: Wed, 1 Oct 2025 09:53:03 -0400
Subject: [PROJ] Geodesics and projections for triaxial ellipsoids
Message-ID: <81b4ffa1-364f-424c-acb3-fd807c844f51@alum.mit.edu>

Version 2.6 of the C++ library GeographicLib

   https://geographiclib.sourceforge.io/C++/2.6/index.html

offers some capabilities for dealing with triaxial ellipsoids which may
be of interest:

* Triaxial::Geodesic3
   A treatment of the direct geodesic problem using the solution given by
   Jacobi (1839) and Liouville (1844).  The inverse problem is solved by
   generalizing the method I used for biaxial ellipsoids.

* Triaxial::Conformal3
   An implementation of Jacobi's conformal projection (1842-1843).  This
   maps lines of constant (ellipsoidal) latitude and longitude into
   straight lines and so is the natural generalization of the Mercator
   projection.  A byproduct is the ability to conformally map a triaxial
   ellipsoid onto a sphere and thence onto any other ellipsoid.

* Triaxial::Cartesian3
   Methods for transforming between various triaxial coordinates
   (cartesian to geodetic, etc.).

I am 3/4 the way through the paper detailing the geodesic problem.  A
writeup of the conformal projection will follow.

-- 
Charles Karney <karney at alum.mit.edu>

From j1 at jimenezshaw.com  Mon Oct  6 12:29:56 2025
From: j1 at jimenezshaw.com (Javier Jimenez Shaw)
Date: Mon, 6 Oct 2025 21:29:56 +0200
Subject: [PROJ] Question about new grid file in Czech Rep.
Message-ID: <CADRrdKs30B6ygydHj64mnr01YAYTSqJvVwqYAgHs=P2xjthzSw@mail.gmail.com>

Hi

Playing with the latest master in the Czech Republic, I noticed something
strange.
The new grid file cz_cuzk_table_-y-x_3_v1710.tif is used only if the
projected CRS EPSG:5514 is used, but not with its geographic
"parent", S-JTSK EPSG:4156. Well, it is at position 13 on the list of
operations.

Just compare
PROJ_NETWORK=ON ./projinfo -o proj --3d EPSG:4258 EPSG:5514+8357
--spatial-test intersects

with

PROJ_NETWORK=ON ./projinfo -o proj --3d EPSG:4258 EPSG:4156+8357
--spatial-test intersects

Doing only 2D transformation, it is even more clear. The grid file is never
used with EPSG:4156

Does it make sense?

I know that the grid file is defined in EPSG:5514. But I assume that the
correction applies to the geographic system transformation, not to the
Krovak projection itself.

Cheers
Javier.
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From even.rouault at spatialys.com  Mon Oct  6 12:55:07 2025
From: even.rouault at spatialys.com (Even Rouault)
Date: Mon, 6 Oct 2025 21:55:07 +0200
Subject: [PROJ] Question about new grid file in Czech Rep.
In-Reply-To: <CADRrdKs30B6ygydHj64mnr01YAYTSqJvVwqYAgHs=P2xjthzSw@mail.gmail.com>
References: <CADRrdKs30B6ygydHj64mnr01YAYTSqJvVwqYAgHs=P2xjthzSw@mail.gmail.com>
Message-ID: <29f266b9-1117-49e7-aeca-6b03994a9d1d@spatialys.com>

Javier,

PROJ is not smart enough to use datum transformations involving a 
projected CRS when doing datum transformations between geographic CRS, 
without an explicit record in the database. So here you would need to 
add a concatenated transformation record chaining 
PROJ:ETRS89_3D_TO_S_JTSK_E_N_BALTIC_HEIGHT with the inverse map 
projection of S-JTSK / Krovak East North EPSG:5514

Looking at the cuzk online transformer at 
https://geoportal.cuzk.cz/(S(aipi30p34gvo5u3w1xywxga3))/Default.aspx?lng=EN&mode=TextMeta&text=wcts&menu=19 
, coordinates in S-JTSK can only be given/returned with the projected 
CRS. Perhaps it is not "idiomatic" to use geographic coordinates in that 
system.

Even

Le 06/10/2025 ? 21:29, Javier Jimenez Shaw via PROJ a ?crit?:
> Hi
>
> Playing with the latest master in the Czech Republic, I noticed 
> something strange.
> The new grid file?cz_cuzk_table_-y-x_3_v1710.tif is used only if the 
> projected CRS EPSG:5514 is used, but not with its geographic 
> "parent",?S-JTSK EPSG:4156. Well, it is at position 13 on the list of 
> operations.
>
> Just compare
> PROJ_NETWORK=ON ./projinfo -o proj --3d EPSG:4258 EPSG:5514+8357 
> --spatial-test intersects
>
> with
>
> PROJ_NETWORK=ON ./projinfo -o proj --3d EPSG:4258 EPSG:4156+8357 
> --spatial-test intersects
>
> Doing only 2D transformation, it is even more clear. The grid file is 
> never used with EPSG:4156
>
> Does it make sense?
>
> I know that the grid file is defined in EPSG:5514. But I assume that 
> the correction applies to the geographic system transformation, not to 
> the Krovak projection itself.
>
> Cheers
> Javier.
>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj

-- 
http://www.spatialys.com
My software is free, but my time generally not.
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From j1 at jimenezshaw.com  Mon Oct 27 11:03:56 2025
From: j1 at jimenezshaw.com (Javier Jimenez Shaw)
Date: Mon, 27 Oct 2025 19:03:56 +0100
Subject: [PROJ] projinfo EGM2008
Message-ID: <CADRrdKtfezzuDytcLAs1FG=pxmFNHSUPAHayL6HvU84VsmUSTQ@mail.gmail.com>

Hi

Today I didn't remember the EPSG code for EGM2008, so I did this:

$ projinfo EGM2008 -k crs
WKT2:2019 string:
VDATUM["EGM2008 geoid",
    ID["EPSG",1027]]

But I was expecting the definition of the CRS, not the Datum:

$ projinfo EPSG:3855 -k crs
PROJ.4 string:
+geoidgrids=us_nga_egm08_25.tif +geoid_crs=WGS84 +vunits=m +no_defs
+type=crs

WKT2:2019 string:
VERTCRS["EGM2008 height",
    VDATUM["EGM2008 geoid"],
    CS[vertical,1],
        AXIS["gravity-related height (H)",up,
            LENGTHUNIT["metre",1]],
    USAGE[
        SCOPE["Geodesy."],
        AREA["World."],
        BBOX[-90,-180,90,180]],
    ID["EPSG",3855]]

Is that a bug? Or did I misunderstand the "-k crs" parameter?

Cheers
Javier
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From even.rouault at spatialys.com  Mon Oct 27 12:36:38 2025
From: even.rouault at spatialys.com (Even Rouault)
Date: Mon, 27 Oct 2025 20:36:38 +0100
Subject: [PROJ] projinfo EGM2008
In-Reply-To: <CADRrdKtfezzuDytcLAs1FG=pxmFNHSUPAHayL6HvU84VsmUSTQ@mail.gmail.com>
References: <CADRrdKtfezzuDytcLAs1FG=pxmFNHSUPAHayL6HvU84VsmUSTQ@mail.gmail.com>
Message-ID: <10f2559b-8e90-4f13-af56-9da6cbf8d2fe@spatialys.com>

Hi Javier,

this will be fixed per https://github.com/OSGeo/PROJ/pull/4592

Even

Le 27/10/2025 ? 19:03, Javier Jimenez Shaw via PROJ a ?crit?:
> Hi
>
> Today I didn't remember the EPSG code for EGM2008, so I did this:
>
> $ projinfo EGM2008 -k crs
> WKT2:2019 string:
> VDATUM["EGM2008 geoid",
> ? ? ID["EPSG",1027]]
>
> But I was expecting the definition of the CRS, not the Datum:
>
> $ projinfo EPSG:3855 -k crs
> PROJ.4 string:
> +geoidgrids=us_nga_egm08_25.tif +geoid_crs=WGS84 +vunits=m +no_defs 
> +type=crs
>
> WKT2:2019 string:
> VERTCRS["EGM2008 height",
> ? ? VDATUM["EGM2008 geoid"],
> ? ? CS[vertical,1],
> ? ? ? ? AXIS["gravity-related height (H)",up,
> ? ? ? ? ? ? LENGTHUNIT["metre",1]],
> ? ? USAGE[
> ? ? ? ? SCOPE["Geodesy."],
> ? ? ? ? AREA["World."],
> ? ? ? ? BBOX[-90,-180,90,180]],
> ? ? ID["EPSG",3855]]
>
> Is that a bug? Or did I misunderstand the "-k crs" parameter?
>
> Cheers
> Javier
>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj

-- 
http://www.spatialys.com
My software is free, but my time generally not.
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