The term indirect fire applies here. It means simply a target that cannot be engaged because of some kind of obscuration such as smoke, darkness or the rise in the ground Peter speaks of. The rise isn't much, just enough, and far enough away that rifle fire and direct fire weapons can't engage the target. I used to teach this very thing in the Infantry School in Gagetown NB. The sight just gives some reference to elevation and direction so several guns may be layed the same.