In fly levelling, if the back sight (BS) staff reading is greater than the fore sight (FS) staff reading, it indicates a ______ in ground level.
Correct Answer: A. Rise
📚 Detailed Explanation: BS > FS Indicates a Rise in Ground Level
Why A (Rise) is correct: The mathematical relationship RL(new) = RL(old) + BS − FS shows that if BS > FS, then (BS − FS) is positive, meaning RL(new) > RL(old). The ground has risen between the two points.
RL of new point = RL of old point + BS – FS
If BS > FS:
(BS – FS) is POSITIVE
RL(new) > RL(old) → RISE in ground level
If BS < FS: (BS - FS) is NEGATIVE RL(new) < RL(old) → FALL in ground level Example: RL(A) = 100.00 m, BS = 2.50 m, FS = 1.20 m RL(B) = 100.00 + 2.50 - 1.20 = 101.30 m → RISE of 1.30 m
Intuitive Understanding
| Situation | Staff Reading | Ground Level Interpretation |
|---|---|---|
| Ground at A is lower | Large BS reading (staff appears high above ground) | Instrument line of sight is far above A |
| Ground at B is higher | Smaller FS reading (staff barely above the line of sight) | B is closer to the line of sight → higher elevation |
| BS > FS | — | B is higher than A → RISE |
| BS < FS | — | B is lower than A → FALL |
- BS > FS → RL increases → Rise in ground level.
- BS < FS → RL decreases → Fall in ground level.
- Rise = BS − FS (when BS > FS); Fall = FS − BS (when FS > BS).