If the object to be viewed is below the level of the observer, then the angle by which the observer lowers his head down is called _____.

A man sitting at a height 20m on a tall tree on a small island in the middle of river observes two poles directly opposite each other on the two banks of river & in a line with the foot of the tree. If the angles of depression of the feet of the poles from a point at which the man is respectively. Find the width of river.