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

Sergio Ibarra-Espinosa (2018) cptcity: incorporating the cpt-city archive into R. R package version 1.0.0. https://CRAN.R-project.org/package=cptcity

Corresponding BibTeX entry:

  @Manual{,
    title = {cptcity: incorporating the cpt-city archive into R},
    author = {Sergio Ibarra-Espinosa},
    year = {2017},
    note = {Colour gradients from
      http://soliton.vm.bytemark.co.uk/pub/cpt-city/},
    url = {https://CRAN.R-project.org/package=vein},
  }