Levelling

Levelling finds the height difference between points. It sets floor levels, road gradients, drain falls and bridge bearing levels, so a small error can make water flow the wrong way.

A dumpy or automatic level gives a horizontal line of sight. Readings are taken on a graduated staff held on each point. The first reading on a point of known level is the backsight; the reading on a point whose level is wanted is a foresight; readings between are intermediate sights. When the instrument is moved, the staff point used for both a foresight and the next backsight is the change point.

Two booking methods are used. The height of instrument method adds the backsight to the known level to get the line-of-sight height, then subtracts each reading. The rise and fall method compares each reading with the one before. Both must satisfy the arithmetic check: sum of backsights minus sum of foresights equals last level minus first level.

Errors are controlled by balancing backsight and foresight lengths, which cancels collimation error and much of the curvature and refraction effect, and by closing every run back on a benchmark.