Tape Measure Home Runs: How accurate?

I often hear home run distances given on baseball game broadcasts. Just how accurate are these distances? The reason I ask is that most home runs don’t complete their trajectories, since they are stopped by seats, signs or other obstructions. Other times, like balls hit out of Fenway Park over the Green Monster probable go further than ground-level at the ballpark. But the measurements are given in feet, implying that the measure is pretty precise. For example, I think one of Hamilton’s Home Run Derby shots was measured at 518 feet, iirc. Clearly the angle of the trajectory is important in determining how far the ball would have flown. Is this factored in?

I did some very quick research and found this: http://entertainment.howstuffworks.com/question704.htm