But doesn't the position of the barrel change all the time? The barrel is freely movable, isn't it?
There is a bubble level at the base of the mortar tube which tells you how much to elevate it for a given range and charge combination when firing in hand-held mode. Direction is determined visually, basically point the tube in the direction of the target. There are no sights attached to the tube in hand-held mode so you're not using stakes or anything like that.