JayUtah
Penultimate Amazing
I recently looked up Dealy Plaza/Elmstreet on Google streetview and maps.
After looking at the many photographs and the film, the first thing you notice when you visit Dealey Plaza in person is how small it is. You can practically spit from Abraham Zapruder's vantage point to the Grassy Knoll.
The distance from the window to the motorcade is not any farther than an infantry rifleman would be expected to shoot. And the motorcade's travel along the line of sight makes for easier sighting. What's harder, in my opinion, is to shoot from the Grassy Knoll essentially across the motorcade's line of travel. It's a much harder to shot to set up, and the slope of the street (which isn't always apparent in photographic records) makes for a shooting window of just a very few seconds.
Granted, I'm not an expert at shooting Presidents, but it doesn't take more than a few minutes at Dealey Plaza to realize how absurd the Grassy Knoll assassin theory is. If you've ever fired a rifle at a moving target (which I have, many times) the choice of sniper's nest is pretty obvious.