Fly Levelling

Surveyor starts the work having a back sight on the bench- mark by setting instrument at a convenient  point If the work site is away from the benchmark,. Then he proceeds towards the site by taking fore sights and until he establishes a temporary benchmark in the site back sights on a number of change points. Rest of the levelling work is performed in the site. At the end of the work by taking a set of convenient change points again levelling is carried out until the bench work is reached. This type of levelling in which only fore sights and back sight are taken, is known fly  levelling, the purpose being to attach a benchmark with a temporary benchmark  or vice versa. Therefore the difference between fly levelling and differential levelling is in the purpose of levelling only.