Não informado. Outdoor Recreation Research. U.S. Department of the Interior, Bureau of Outdoor Recreation.
Παραπομπή σε μορφή Chicago (17η εκδ.)Não informado. Outdoor Recreation Research. Washington: U.S. Department of the Interior, Bureau of Outdoor Recreation.
Παραπομπή σε μορφή MLA (8th εκδ.)Não informado. Outdoor Recreation Research. U.S. Department of the Interior, Bureau of Outdoor Recreation.
Πρόσοχή: Οι παραπομπές μπορεί να μην είναι 100% ακριβείς.