Trailing dogs are trained to follow a specific human scent, which may or may not approximate the path the person took because of factors affecting the dispersal of scent such as wind and temperature. To start the dog is started on the trail at the point where the victim was last seen (PLS). Trailing dogs will follow the route of scent deposited on the ground as a person moves through an area. A trained trailing dog can follow the steps of someone who passed by several days earlier, discriminate between it and another's trail, and follow it over hills and through marshland. Dogs can even trail people in cars, from the scent that blows out of the window or through the vents of the car.