If you want to cite the cptcity R package, please use:

Ibarra-Espinosa S (2024). cptcity: incorporating the cpt-city archive into R. Colour gradients from http://seaviewsensing.com/pub/cpt-city/, https://CRAN.R-project.org/package=vein.

Corresponding BibTeX entry:

  @Manual{,
    title = {cptcity: incorporating the cpt-city archive into R},
    author = {Sergio Ibarra-Espinosa},
    year = {2024},
    note = {Colour gradients from
      http://seaviewsensing.com/pub/cpt-city/},
    url = {https://CRAN.R-project.org/package=vein},
  }