That line is the foul line. There are no baselines marked on the field. The runner establishes the baseline (as long as he does not create a "travesty of the game"). The runner is out if he runs more than three feet out of the baseline when avoiding being tagged (NF 8-4-2). I learned at school that a good way to judge the 3-foot rule is: If the defensive player with the ball moves "a step and a reach" toward the runner when attempting a tag -- but the runner moves more than "a step and a reach" (and is not tagged) -- then the runner has left the baseline and is out.