Sure, simple curvature formula... Kinda like how in high school you calculate velocity by ignoring friction.
In this case the formula is ignoring elevation. Notice that at around 3 minutes the cameraman is on an mountain? Look at the island then, around 3:15 you can clearly see ground where the gap is I mentioned before. In fact, along the entire island you can see more of the mountain.
Because being above the curve lets you see further along the curve, but OP's original post basically assumes everything is exactly on the surface (e.g. sea level.)