What does the sentence "the ideal viewer was on the ground with a stable horizon line" appears in the fourth line from the botom mean?

Around the time Leonardo Da Vinci was painting the Mona Lisa, he was also writing his Codex on the Flight of Birds, a roughly 35,000-word exploration of the ways in which man might take to the air. His illustrations included diagrams positing pre-Newtonian theories of physics, a rudimentary plan for a flying machine and many, many sketches of birds in flight. The Mona Lisa, with her secretive smile, is a universe of intimacy captured on a relatively small panel of wood. But the landscape behind his captivating subject shows the world as you would see it from atop a tall hill—or from the vantage point you would get if you had hitched a ride on the back of a giant bird. Even as da Vinci was perfecting one way of seeing a face, he was dreaming of other ways of looking. No wonder he wanted to fly, perhaps less for the physical rush than for the thrill of seeing the world from above.

That’s the pleasure drones give us: they send eyes where our bodies can’t easily go unencumbered. A GoPro camera attached to a bird of prey shows us where the bird wants to go, which clues us in to what it’s thinking. Drones, as of now operable only by humans, tell us what humans find visually interesting. Drones are practical, but like any tool wielded by humans—pencil or paintbrush or maestro’s baton—there’s poetry in them too. Because of this, more and more, drones are finding their way into the art world.

“If you think about traditional art and Renaissance perspective, the ideal viewer was on the ground with a stable horizon line,” says Matthew Biro, a professor of modern and contemporary art at the University of Michigan. “And the drone takes us off that. It takes us out of our body in a certain way, kind of giving us an overlaid perspective.”