How was the aerial prison scene shot?

The chopper does not really come all that close to the building. And yes, *it is a chopper, baby*.

The following is apparently from director Frank Darabont's notes:

Scene 10. This scene contains everybody's favorite shot in the movie. It's the amazing aerial view that first reveals the prison in all its bleak glory, with the army of prisoners streaming across the yard below to greet the arriving bus. It's a wonderfully cinematic moment, one that seems to suspend time even as it plunges us breathlessly into this new and horrible world...

...and boy, would I love to take credit for it, but you'll notice it's not even alluded to in the script. Truth is, it was production designer Terence Marsh's idea. On our very first scout to determine the viability of using the abandoned Ohio State Reformatory in Mansfield as our primary location, producer Niki Marvin, coexecutive producer David Lester, Terry Marsh, and I found ourselves abjectly freezing our butts off and picking sleet out of our teeth (real winters come as a nasty surprise if you've just flown in from California). Suddenly, Terry (two-time Academy Award winner and last of the soft-spoken gentlemen) blinked up at the sky and uttered something like: "This place would look smashing with an opening helicopter shot."

Six months later we were back again, this time with a veritable army of crew and trucks, desperately trying to get the shot in the can before lunch. This involved coordinating three major elements (and believe me, everybody's timing had to be perfect): the helicopter in the air, the bus on the ground, and the 500 extras in the yard. It didn't help that the copter could only go up (or the extras emerge from hiding) in the intermittent breaks between rain flurries. The shot came off perfectly. It's even got the state flag of Maine snapping smartly in the breeze as if saluting Stephen King. It helps to have a great pilot (Bobby Z), a great aerial camera operator (Mike Kelem), intrepid assistant directors (John Woodward and Tom Schellenberg, who spent a month planning placement and movement of the prisoners in the yard as if diagramming the world's biggest football play), stout-hearted extras (the fine men of Mansfield and surrounding areas), and God or the universe on your side. Most of all, it helps to have a great idea to set all the madness in motion. Thanks, Terry.

The use of the helicopter is also confirmed in a goof:

Towards the beginning of the film, during a beautiful aerial shot of the bus entering Shawshank Prison, the camera flies over the buildings, where we see the prisoners on their way to "greet" in the new inmates. As the camera circles around, at the top of the frame, in an area of green grass, near a building, the shadow of the camera's helicopter is clearly visible.