Russell Bradley, M.D., and Keith Conover, M.D., FACEP set out a very basic orienteering course in the Green Tree Nature Center off Green Tree Road just off I-279. (Take Green Tree Road from I-279, the Parkway West, past where the blue belt turns left on Potomac Avenue, about a mile.  A couple of blocks later turn right on Crestmont at the top of the hill and park at the end of the street.) Using the Orienteering Course Map and the following description along with ASRC Grid coordinates, you can run the course to test your land navigation ability.  Each control point is marked with a 5.5"x8.5" laminated marker, white and orange, with a letter on it.  Your job is to find each of the markers and note down the letter mark on it. For extra credit, figure out which marker is marked differently than the others (it's subtle).