United States Geological Survey (USGS). (n.d.). 1:250,000 Digital Raster Graphics for Maryland. State of Pennsylvania. http://www.pasda.psu.edu/uci/DataSummary.aspx?dataset=620

Available from: https://geo.btaa.org/catalog/921a3f23-dd2f-4487-832b-6b35f4b29f5a