In a helicopter above the city on Friday, Stephen Wiltshire of London looked down at the streets and sprawl of New York. He flew for 20 minutes. Since then, working only from the memory of that sight, he has been sketching and drawing a mighty panorama of the city, rendering the city’s 305 square miles along an arc of paper that is 19 feet long. He is working publicly in a gallery at the Pratt Institute in Brooklyn.

“I always memorize by helicopter,” he said on Tuesday, pausing from detailing the corners of a street on the Brooklyn side of the Williamsburg Bridge.

Mr. Wiltshire sees and draws. It is how he connects. Until age 5, he had never uttered a word. One day, his kindergarten class at a school for autistic children in London went on a field trip.

When they came back, he spoke.

“He said, ‘Paper,’ ” his sister, Annette Wiltshire, said. “The teacher asked him to say it again. He said it. Then they asked him to say something else, and he said, ‘Pen.’ ”