Q10. Which of the following represents the CORRECT order of setting up a plane table?
📚 Detailed Explanation: The Correct Plane Table Setup Sequence
Setting up a plane table correctly requires three operations in a specific logical order. Getting the sequence wrong leads to errors that are difficult to correct and may invalidate the subsequent survey observations.
Fig: The three-step plane table setup sequence — always Level first, then Center, then Orient.
Why This Sequence is Correct
Step 2 — Centering: Once the table is level, the plumbing fork is used to position the station point on the drawing sheet directly above the physical ground peg. Centering must come after levelling because adjusting the tripod legs for levelling can shift the table’s horizontal position, undoing any previous centering.
Step 3 — Orientation: Finally, the table is rotated (without disturbing level or center) so that the lines on the sheet are parallel to the corresponding lines on the ground. Orientation is done last because any subsequent levelling or centering adjustment would rotate the table and destroy the achieved orientation.
Why the Other Options Are Wrong
| Option | Sequence | Problem |
|---|---|---|
| A | Center → Level → Orient | Centering before levelling: adjusting the table for level will shift its position, requiring re-centering. Centering over a tilted table also has no meaning because the plumb bob direction differs from vertical. |
| B | Center → Orient → Level | Orienting before levelling: tilted table makes orientation meaningless since sighting directions are not horizontal. Any levelling adjustment after orientation also destroys it. |
| D | Level → Orient → Center | Orienting before centering: centering adjustment requires rotating/shifting the table, which would undo the achieved orientation. Order must be Level → Center → Orient. |
Key Concepts for Students
- Memory trick — “LCO”: Level → Center → Orient. Think: “Level the ground, Center the point, Orient the sheet.” This exact sequence (C) appears consistently in all standard Indian surveying textbooks and exam papers.
- Levelling must come first because everything else depends on it: A tilted table makes centering inaccurate (the plumb bob hangs at an angle relative to the table) and makes orientation meaningless (sighted rays are not in a horizontal plane). Always level before doing anything else.
- Option D (Level → Orient → Center) is the common wrong choice: Students sometimes think orientation should come before centering. The issue is that centering shifts the table position, which in turn slightly rotates it, destroying orientation. Orient last so the final adjustment does not disturb it.