During a levelling survey, the backsight of A was found greater than the foresight reading at point B. Which of the following is correct?
- AElevation of B is more than that of A.Correct
- BElevation of A is more than that of B.
- CHeight of instrument is required to know which point is higher.
- DEither A or B can have higher elevation.
Solution
Concept
A smaller staff reading means a higher point, because the line of sight is at one fixed level.
Formula
RLB = RLA + BSA - FSB
Step-by-step solution
1. Height of instrument: HI = RLA + BSA. 2. RLB = HI - FSB = RLA + BSA - FSB. 3. Given BSA > FSB, so BSA - FSB is positive. 4. So RLB > RLA. Point B is higher than A.
Explanation
The line of sight of the level is horizontal at one height. The staff reading at A is larger, so A is further below the line of sight. So B is higher than A.
Why the other options are wrong
B: A has the larger reading, so A is lower, not higher. C: HI is not needed. The difference of readings alone shows the rise or fall. D: The result is fixed. B is always higher when BSA > FSB.
Shortcut
Bigger reading = lower point. BS > FS means rise.
Common mistake
Thinking that a bigger staff reading means a higher point.
Correct answer: A) Elevation of B is more than that of A.