To cite elevatr:

Hollister, J.W., Tarak Shah (2017). elevatr: Access Elevation Data from Various APIs.

Corresponding BibTeX entry:

  @Manual{,
    author = {Jeffrey Hollister and {Tarak Shah}},
    title = {elevatr: Access Elevation Data from Various APIs},
    year = {2017},
    note = {R package version 0.1.3, doi:10.5281/zenodo.400259},
    url = {http://github.com/usepa/elevatr},
  }