ok imagine you are on one of those lifts at the seaside that take you up and down the cliff. You have a gantry across it when it is at the top of the hill and you level the roof off. You decide to move it along the rails a bit, is you table at the same height then? in you case it is the same except that on side of the rails is square with the gantry and the other side is on it's way down the hill.