Points to consider in fly levelling

Fly levelling is levelling in which only BS and FS readings are taken, to find the elevation of a distant BM or to check levels previously established. The points to be considered are:

1. Balance the sights: keep the BS and FS distances approximately equal to remove the errors of collimation, curvature and refraction.
2. Sight length: limit the length of sight to 60–100 m (usually not more than 100 m) and avoid very short sights; it should be such that the staff can be read with confidence.
3. Change points: choose firm, well-defined points (such as pegs with a nail or a stone) and keep the staff on the same spot between the FS and the BS; the staff should be held vertical (use a staff bubble).
4. Starting and closing: start from a BM of known RL and close on another BM, or return to the starting BM (a loop), to check the work.
5. Readings should be taken to the nearest millimetre, entered at once in the field book and checked: $\Sigma BS - \Sigma FS =$ last RL $-$ first RL.
6. Instrument: it must be well adjusted (two-peg test); the bubble should be centred before each reading; avoid sighting at the bottom (< 0.5 m) of the staff because of refraction, and avoid reading in strong wind or heat shimmer.
7. Closing error must be within the permissible limit, $\pm C\sqrt{K}$ mm (K in km), for example $\pm 24\sqrt{K}$ for ordinary work and $\pm 12\sqrt{K}$ for more accurate work; if exceeded, repeat the levelling.
8. The work should be done quickly, in one direction, with the same instrument and observer, and in the same weather conditions.