For one thing you need to know how far away the object is and if there's any difference in elevation. You can use the stick method. You put a stick in the ground where you are, step away, line up line of sight from the stick and measure the angle. Basically use another object you know the height of to create a right triangle. I don't think there is a specialized app for that, but you don't need one, just basic geometry, pencil, paper, and a calculator.