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I agree with the others that it's stupid to have a line painted on a wall so that if the ball hits above the line it is a homerun. Just set the height of the wall where you want a home run to be.
Didn't some old ball parks have a line with metal sheathing above the the home run line? That way, there was a distinctive sound when the ball hit above the line. I seem to remember one park where this sheathing stuck out from the fence and had a beveled edge. When the ball hit the sheathing near the edge, the beveled angle made the ball bounce off the wall in such a way to make it obvious that it had hit above the line.
If there are going to be stadiums with this stupid convention, it ought to be possible to figure a way to determine for sure whether the ball hit above the line without going to instant replay.