Penn State Institutes of Energy and the Environment. (2013). Pennsylvania Spatial Data Access. Retrieved from . State of Nebraska. (2013). NebraskaMap. Retrieved from . State Library of Iowa. (2014). State Data Center of Iowa. Retrieved from . U.S. Department of Agriculture Natural Resources Conservation Service. (2014, March 11). Geospatial Data Gateway. Retrieved from . U.S. Geological Survey. (2014a, May 14). EarthExplorer. Retrieved from . U.S. Geological Survey. (2014b, May 13). The National Map. Retrieved from /.
Penn State Institutes of Energy and the Environment. (2013). Pennsylvania Spatial Data Access. Retrieved from . State of Nebraska. (2013). NebraskaMap. Retrieved from . State Library of Iowa. (2014). State Data Center of Iowa. Retrieved from . U.S. Department of Agriculture Natural Resources Conservation Service. (2014, March 11). Geospatial Data Gateway. Retrieved from . U.S. Geological Survey. (2014a, May 14). EarthExplorer. Retrieved from . U.S. Geological Survey. (2014b, May 13). The National Map. Retrieved from /.