Porta, J. and Thomas, F. (2009). Concise Proof of Tienstra’s Formula.J. Surv. Eng., 135(4), 170–172. Retrieved February 2015
books.google.com
Philip Howard (2006) Archaeological Surveying and Mapping: Recording and Depicting the Landscapepage 51 Routledge ISBN1134400861 Retrieved February 2015