For which of the following applications in levelling is an inverted staff reading most suitable?
- ALevelling across a steep slope
- BLevelling across a lake
- CLevelling across a wallCorrect
- DLevelling across an intervening, high or low ground
Solution
Concept
An inverted staff reading is taken when the point is above the line of sight, for example the top of a high wall, a ceiling, or a bridge soffit.
Formula
RL of point = HI + inverted staff reading
Step-by-step solution
1. An inverted staff is used when the point is higher than the line of collimation. 2. When levelling across a high wall, the top of the wall is above the instrument's line of sight. 3. The staff is held upside down on the wall top, and the reading is recorded as negative. 4. So the inverted staff reading is most suitable for levelling across a wall.
Explanation
When the point is above the line of sight, the staff is held upside down against it. The top of a high wall is a typical such point. Its RL is found by adding the reading to HI.
Why the other options are wrong
A: A steep slope is handled with frequent change points or shorter sights. B: A lake is crossed with reciprocal levelling or the water surface. D: Intervening high or low ground is handled with change points.
Shortcut
If the point is above the line of sight (wall top, roof, soffit), think inverted staff.
Common mistake
Subtracting the inverted reading from HI instead of adding it.
Correct answer: C) Levelling across a wall