http://memory.loc.gov/cgi-bin/query/r?ammem/gmd:@field(NUMBER+@band(g4126p+mf000065))
http://www.fhwa.dot.gov/infrastructure/ppmap.htm
http://www.dot.state.mn.us/library/reg_trails.html
http://www.marion.ohio-state.edu/fac/schul/trails/trails.html