To cite PROJ in publications use:

  PROJ contributors (2020). PROJ coordinate transformation software
  library. Open Source Geospatial Foundation. URL https://proj.org/.

A BibTeX entry for LaTeX users is

.. code-block:: latex

  @Manual{,
    title = {{PROJ} coordinate transformation software library},
    author = {{PROJ contributors}},
    organization = {Open Source Geospatial Foundation},
    year = {2021},
    url = {https://proj.org/},
  }
