Please cite fields including its version and DOI as

Douglas Nychka, Reinhard Furrer, John Paige and Stephan Sain (2015). “fields: Tools for spatial data.” doi: 10.5065/D6W957CT, R package version 9.0, www.image.ucar.edu/fields.

Corresponding BibTeX entry:

  @Misc{,
    title = {fields: Tools for spatial data},
    author = {{Douglas Nychka} and {Reinhard Furrer} and {John Paige}
      and {Stephan Sain}},
    note = {R package version 9.0},
    organization = {University Corporation for Atmospheric Research},
    address = {Boulder, CO, USA},
    year = {2015},
    url = {www.image.ucar.edu/fields},
    doi = {10.5065/D6W957CT},
  }