Determining the difference of elevation between two points by taking foresights to leveling rods on both points from one set up and backsights to both leveling rods from a second set up. The method is used principally when it is not feasible to level between the two points by the ordinary method of differential leveling, as when the points are separated by a wide river or a steep valley. The method eliminates errors caused by the curvature or the Earth or by lack of horizontality of the line of sight but does not usually eliminate all of the error caused by refraction.