The United States Geological Survey provides guidelines for citing USGS data products (as downloaded from 'get_tiles') at: https://www.usgs.gov/faqs/how-should-i-cite-datasets-and-services-national-map To cite terrainr in publications please use:

Mahoney M (2021). terrainr: Landscape Visualizations in R and Unity. doi: 10.5281/zenodo.5142763, R package version 0.5.0, https://CRAN.R-project.org/package=terrainr.

Corresponding BibTeX entry:

  @Manual{,
    title = {{terrainr}: Landscape Visualizations in R and Unity},
    author = {Michael Mahoney},
    year = {2021},
    note = {R package version 0.5.0},
    doi = {https://doi.org/10.5281/zenodo.5142763},
    url = {https://CRAN.R-project.org/package=terrainr},
  }