@misc{indiciaedac06a08de28,
  title = {The multi-slope model: A constructed stereoscopic model showing angles of slope from 2 to 90 degrees at different locations and sloping in different directions in the model},
  author = {Robert J. Hackman},
  year = {1956},
  doi = {10.3133/ofr5648},
  url = {https://pubs.usgs.gov/publication/ofr5648},
  note = {Source identifier: ofr5648}
}