Pump station No. 4 has 2 unique qualities; first, it has the highest elevation of all 11 stations stretching from Prudhoe Bay to Valdez, sitting at 2760 feet. Second, this is one of the launching and receiving stations for devices called pigs. Pump Station 1 is also a launch point. There are two types of pigs, named for the squealing sound they make as they move through the Trans Alaska Pipeline. A cleaning or dump pig travels through the pipeline cleaning any build-up on the inside. An instrumented or smart pig is equipped with sensors that scan in inside of the pipe checking the condition of the walls and monitoring for any maintenance needs. The data collected from the smart pig is used by engineers to fix or replace any weak areas along the pipeline before they become a problem.