What is the correct sequence of the temporary adjustment of a level instrument at a new field station?
Correct Answer: C. Setting, centering and leveling
📚 Detailed Explanation: Correct Sequence of Temporary Adjustment = Setting, Centering, Levelling
Why C (Setting, centering and levelling) is correct: Temporary adjustments are performed every time the instrument is set up at a new station. The logical sequence is: (1) set up the tripod, (2) centre the instrument over the station mark, (3) level the instrument so the bubble is central.
Sequence of Temporary Adjustments
| Step | Action | Purpose |
|---|---|---|
| 1. Setting up | Plant the tripod firmly over the station; attach the instrument to the tribrach | Establishes a stable base at the correct location |
| 2. Centering | Adjust the tripod legs and/or use the optical plummet to position the instrument exactly over the ground station mark | Ensures the instrument's vertical axis passes through the station point |
| 3. Levelling | Use foot screws (and the plate bubble or circular bubble) to make the vertical axis truly vertical | Ensures horizontal plane of rotation and level line of sight |
| 4. Focussing (elimination of parallax) | Focus eyepiece to see cross hairs sharply; focus objective lens for a clear staff image | Removes apparent cross hair movement relative to staff when eye is moved |
Memory Aid: “SCLaF” — Setting, Centering, Levelling, and Focussing. The first three are the “temporary adjustments”; focussing is done before each observation.
- Correct sequence: Setting up → Centering → Levelling (+ focussing before each reading).
- Centering must precede levelling: levelling the instrument can shift the plummet slightly, so centering is re-checked after.