Principles of Differential Leveling

Figure 26 illustrates the principle of differential leveling. The instrument shown in the center is an engineer’s level. This optical instrument provides a perfectly level line of sight through a telescope, which can be trained in any direction. Point A in the figure is a bench mark having a known elevation of 365.01 feet. It could be a concrete monument, a wooden stake, a sidewalk curb, or any other object. Point B is a ground surface point whose elevation needs to be determined.