Obstacle Departure Procedures (ODP’s)—Printed either textually or graphically, ODP’s provide obstruction clearance via the least onerous route from the terminal area to the appropriate en route structure. ODP's may be flown without ATC clearance unless an alternate departure procedure (SID or radar vector) has been specifically assigned by ATC. Unless specified otherwise, required obstacle clearance for all published departures is based on the pilot crossing the departure end of the runway at a minimum of 35 feet above the runway elevation, climbing to 400 feet before making the initial turn, and maintaining a minimum climb gradient of 200 feet per nautical mile (unless required to level off by a crossing restriction). A greater climb gradient may be specified in order to clear obstacles or to achieve an ATC crossing restriction. When ATC has not used the term "at pilot's discretion" nor imposed any climb or descent restrictions, pilots should initiate a climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1500 fpm until the assigned altitude is reached. ODP's will be listed by airport in IFR Take-Off Minimums and Departure Procedures, Section C of the Terminal Procedures Publications (TPPs).