Inspections are made by line walkers, vehicles, and light aircraft. Air patrols should be flown not less than once every three weeks at an elevation of less than 152 meters (500 feet) from the ground and at speeds from 104 to 128 kilometers per hour (65 to 80 miles per hour). The pipeline should be marked with posts or signs at 1.6-kilometer (1-mile) intervals and at bends. The pilot acts as an observer who checks for unnatural changes in vegetation color and oil slicks on lakes and streams which are evidence of leaking pipelines; area construction work (e.g., roads, sewers) that could cross and possibly damage the pipeline; and the overall condition of the right-of-way. Line walkers or vehicle patrols make detailed inspections once a year of the entire pipeline, checking the general condition of the right-of-way, valves in remote areas, supports on above ground pipelines, and any condition that may indicate a leak.
