To cite package ‘proj4’ in publications use:

Urbanek S (2025). proj4: A simple interface to the PROJ.4 cartographic projections library. R package version 1.0-15, https://github.com/s-u/proj4.

Corresponding BibTeX entry:

  @Manual{,
    title = {proj4: A simple interface to the PROJ.4 cartographic
      projections library},
    author = {Simon Urbanek},
    year = {2025},
    note = {R package version 1.0-15},
    url = {https://github.com/s-u/proj4},
  }