Which of the following expressions is used to calculate the R.L. of a point using the Rise and Fall method?
Correct Answer: D. Reduced level of previous point minus fall
📚 Detailed Explanation: RL of a Point = RL of Previous Point Minus Fall (Rise and Fall Method)
Why D (Reduced level of previous point minus fall) is correct: In the Rise and Fall method, the new RL is calculated by either adding the Rise or subtracting the Fall from the previous RL. When the terrain falls (reading increases), the difference is a Fall, and RL(new) = RL(previous) − Fall.
Rise and Fall Formulas
| Situation | Rise or Fall | RL Calculation |
|---|---|---|
| Reading decreases (ground rose) | Rise = Previous reading − Current reading | RL(new) = RL(prev) + Rise |
| Reading increases (ground fell) | Fall = Current reading − Previous reading | RL(new) = RL(prev) − Fall |
Why Other Options Are Wrong
| Option | Expression | Why Wrong |
|---|---|---|
| A. HI minus BS | HI − BS | This gives the RL of the BM station from which the HI was established — that's RL = HI − BS rearranged; not a standard direct expression for any point's RL |
| B. HI plus FS | HI + FS | Never correct; RL = HI − FS (minus, not plus) |
| C. RL of previous minus FS | RL(prev) − FS | Confuses Rise and Fall method with HI method; this is not a standard formula |
| D. RL of previous minus Fall | RL(new) = RL(prev) − Fall | CORRECT — standard Rise and Fall method formula |
- RL(new) = RL(previous) − Fall (when terrain drops).
- RL(new) = RL(previous) + Rise (when terrain rises).
- This is a direct formula for the Rise and Fall reduction method.