Optical transits and dumpy levels are commonly used in surveying to determine the difference in elevation of two points as well as estimating distances. The idea behind determining elevation is rather simple, you level the unit then measure how high the level is above two separate points and the difference of the values is the difference in elevation. To estimate a distance you put a grade rod at the end point, and then you use the rod to measure the distance between the stadia lines, or the smaller crosshairs on the transit. The industry standard is that the distance between the stadia lines is 1/100 of the distance to the point. Some transit and dumpy level models are self leveling and others can even check how plumb posts are.