The Zapruder film runs at 18.3 frames a second, and the argument about what happened in Dealey Plaza is conducted almost entirely in frame numbers. These are the passages the book pins to a specific frame, in order. Every frame here is also an address: /frames/#z313 goes straight to the head shot.
The geometry behind them comes from the 1964 survey rather than from the film, which is what lets a frame number carry a distance and an angle. How this was built sets out the method, and the model shows the ground.