What determines hill/cliff bonus, the units location when it launches its attack or when its attack hits?

In other words, if I shoot with a bombard cannon from bottom of hill, and then move it onto hill before the cannonball lands, will the cannonball hit with the elevation bonus? Similarly, if I shoot a cannonball from a bombard cannon on a hill, but the BBC dies before the cannonball hits, will the ball lose its elevation bonus?

I don't have access to game atm, but I suspect when an attack lands on a unit then the game quickly looks at the locations of attacker/attacked to determine any bonuses, and if true then that means you could potentially micro to give your siege an elevation bonus against buildings just slightly out of range by launching attack at building, then moving onto hill, and repeating.