A compressor station is a facility which helps the movement of gas from one location to another in a pipeline system. Gases typically transported over long distances in this way include natural gas, methane, ethylene, hydrogen, ammonia and carbon dioxide. While being transported through a pipeline the gas pressure gradually decreases due to frictional effects and needs to be periodically repressurized at intervals of about 40 to 100 miles. Siting of compressor stations is dependent on terrain, accessibility and whether there are other tie-ins such as gas wells in the vicinity. Frequent elevation changes and a greater number of gas wells will require more compressor stations.