Ground rule doubles

Question
In a couple of ballparks of my experience,  balls hit to dead center filed which, had they been either to left or right would have been homer were called ground rule doubles.  It appears to us as fans that the ball had traveled well over the eight foot high wall and over 410 feet;  but, of course, the ball hit the blank green "visibility" wall and did rebound back onto the field.  Why would a long ball which would otherwise have been an unqualified homerun be a ground rule double?

Answer
Ned,

It is all about the rules that have been set for that particular ballpark.  There are no standard 'ground rules' in baseball.

Depending on the park in question, the visibility wall you mentioned may be considered in play.  A hit isn't considered a home run unless it clears that wall.  Kind of like hitting a home run in Boston verses another park.  The green monster is relatively close to home plate but very tall.  A hard hit line drive off the top of the monster might be a homerun in another park (while being in play here) while an easy high fly might land beyond the wall in Boston but be an easy catch somewhere else.

So, each particular park sets the ground rules and the umpires try and legislate to those rules.

Hope this helps!

Brian