Transformer with MercatorAConversion with lat/lon of natural origin returns ProjError

[kdpenner]

latlon_crs = CRS("EPSG:4326")
conv = MercatorAConversion(latitude_natural_origin=0.1, longitude_natural_origin=0.1)
merc_crs = ProjectedCRS(conversion=conv)
transform_func = Transformer.from_crs(latlon_crs, merc_crs, always_xy=True).transform

Problem description

The above code returns ProjError: Input is not a transformation. If latitude_natural_origin=0 and longitude_natural_origin=0, no error is returned.

Expected Output

No error.

Environment Information

• Output from: pyproj -v

pyproj info:
    pyproj: 3.3.1
      PROJ: 9.0.0
  data dir: /Users/kpenner/dev/pyproj/share/proj
user_data_dir: /Users/kpenner/Library/Application Support/proj
PROJ DATA (recommended version): 1.9
PROJ Database: 1.2
EPSG Database: v10.054 [2022-02-13]
ESRI Database: ArcMap 12.9 [2022-02-18]
IGNF Database: 3.1.0 [2019-05-24]

System:
    python: 3.10.4 | packaged by conda-forge | (main, Mar 24 2022, 17:45:10) [Clang 12.0.1 ]
executable: /Users/kpenner/dev/pyproj/bin/python
   machine: macOS-12.4-x86_64-i386-64bit

Python deps:
/Users/kpenner/dev/pyproj/lib/python3.10/site-packages/_distutils_hack/__init__.py:30: UserWarning: Setuptools is replacing distutils.
  warnings.warn("Setuptools is replacing distutils.")
   certifi: 2022.05.18.1
    Cython: None
setuptools: 62.3.2
       pip: 22.1.2

Installation method

• conda

Conda environment information

Environment (conda list):

# Name                    Version                   Build  Channel
proj                      9.0.0                h2364a93_1    conda-forge
pyproj                    3.3.1           py310h6c09501_0    conda-forge

Details about conda and system ( conda info ):

     active environment : pyproj
    active env location : /Users/kpenner/dev/pyproj
            shell level : 2
       user config file : /Users/kpenner/.condarc
 populated config files : /Users/kpenner/.condarc
          conda version : 4.12.0
    conda-build version : not installed
         python version : 3.7.12.final.0
       virtual packages : __osx=10.16=0
                          __unix=0=0
                          __archspec=1=x86_64
       base environment : /Users/kpenner/miniconda3  (writable)
      conda av data dir : /Users/kpenner/miniconda3/etc/conda
  conda av metadata url : None
           channel URLs : https://conda.anaconda.org/conda-forge/osx-64
                          https://conda.anaconda.org/conda-forge/noarch
                          https://repo.anaconda.com/pkgs/main/osx-64
                          https://repo.anaconda.com/pkgs/main/noarch
                          https://repo.anaconda.com/pkgs/r/osx-64
                          https://repo.anaconda.com/pkgs/r/noarch
          package cache : /Users/kpenner/miniconda3/pkgs
                          /Users/kpenner/.conda/pkgs
       envs directories : /Users/kpenner/dev
                          /Users/kpenner/miniconda3/envs
                          /Users/kpenner/.conda/envs
               platform : osx-64
             user-agent : conda/4.12.0 requests/2.27.1 CPython/3.7.12 Darwin/21.5.0 OSX/10.16
                UID:GID : 501:20
             netrc file : None
           offline mode : False

[snowman2]

projinfo -s 'PROJCRS["undefined",BASEGEOGCRS["undefined",DATUM["World Geodetic System 1984",ELLIPSOID["WGS 84",6378137,298.257223563,LENGTHUNIT["metre",1]],ID["EPSG",6326]],PRIMEM["Greenwich",0,ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8901]]],CONVERSION["unknown",METHOD["Mercator (variant A)",ID["EPSG",9804]],PARAMETER["Latitude of natural origin",0.1,ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8801]],PARAMETER["Longitude of natural origin",0.1,ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8802]],PARAMETER["Scale factor at natural origin",1,SCALEUNIT["unity",1],ID["EPSG",8805]],PARAMETER["False easting",0,LENGTHUNIT["metre",1],ID["EPSG",8806]],PARAMETER["False northing",0,LENGTHUNIT["metre",1],ID["EPSG",8807]]],CS[Cartesian,2],AXIS["(E)",east,ORDER[1],LENGTHUNIT["metre",1,ID["EPSG",9001]]],AXIS["(N)",north,ORDER[2],LENGTHUNIT["metre",1,ID["EPSG",9001]]]]' -t 'EPSG:4326' -o PROJ
Candidate operations found: 1
-------------------------------------
Operation No. 1:

unknown id, Inverse of unknown, 0 m, unknown domain of validity

PROJ string:
Error when exporting to PROJ string: Unsupported value for Latitude of natural origin

[snowman2]

Related: https://proj.org/operations/projections/merc.html

[snowman2]

>>> MercatorAConversion(longitude_natural_origin=10).to_proj4()
'+proj=merc +lon_0=10 +k=1 +x_0=0 +y_0=0'
>>> MercatorAConversion(latitude_natural_origin=10).to_proj4()

[snowman2]

It seems that the PROJ only supports latitude_natural_origin=0.

[snowman2]

Yes, that does seem to be the case:
https://github.com/OSGeo/PROJ/blob/f7999f2b976612551ca632fa8f956498462637c8/src/iso19111/operation/conversion.cpp#L3704-L3712

[snowman2]

Related to #1036, would be good to raise an exception when exporting to PROJ string if NULL is returned.

[kdpenner]

What's the reason for keeping latitude_natural_origin as a kwarg if only 0 is supported?

On an unrelated note, the lat_ts PROJ parameter is unsupported by pyproj.

[snowman2]

What's the reason for keeping latitude_natural_origin as a kwarg if only 0 is supported?

For completeness sake when creating CRS and exporting to WKT. This may be needed even if PROJ does not support it for transformations.

On an unrelated note, the lat_ts PROJ parameter is unsupported by pyproj.

See MercatorBConversion

[snowman2]

Other helpful references:

• https://epsg.org/coord-operation-method_9805/Mercator-variant-B.html
• https://epsg.org/coord-operation-method_9804/Mercator-variant-A.html

[snowman2]

https://epsg.org/coord-operation-method_9804/Mercator-variant-A.html

Note: For the Mercator variant A (1SP) method, in the EPSG Dataset, to be fully transparent about the location of the projection origin, the parameter "latitude of natural origin (LatO)" is included in the defining parameters of the map projection method and map projections. It must have a value of zero because by definition the location of the natural origin for this method is on the equator. However this parameter is not used in the conversion formulas.

Seems like it should always be 0. I am thinking a check on latitude_natural_origin when building the CRS to ensure it is 0 makes sense.

[kdpenner]

Makes sense to me. I'd also note the constraint in the documentation.

[snowman2]

Sounds good to me. A PR with the change is welcome.

[kdpenner]

Happy to submit a PR soon. Should a warning be raised if a non-zero latitude_natural_origin is passed?

[snowman2]

Happy to submit a PR soon. Should a warning be raised if a non-zero latitude_natural_origin is passed?

Thanks! I am thinking since it is invalid, a CRSError should be raised in the initialization around here.