Q3. In plane table surveys, the orientation of the table only by back-sighting is preferred when:
📚 Detailed Explanation: When Back-Sighting is the Preferred Orientation Method
Orientation means rotating the plane table horizontally at each new station so that lines on the drawing sheet remain parallel to the corresponding lines on the ground. Two main methods are used: by trough compass (quick, magnetic-north based) and by back-sighting (precise, geometry-based). The question asks when back-sighting is specifically preferred.
Why Option C is Correct
Back-sighting works only when the current station already has a precisely plotted position on the sheet (because you need to lay the alidade along a known plotted direction). When the plane table is set up over a station that is already plotted from a previous instrument position, you can back-sight to that previous station to orient the table with superior accuracy. This is the preferred method at all stations after the first one in a traverse.
Why the Other Options Are Wrong
| Option | Reason it is wrong |
|---|---|
| A — Traverse is too long | Traverse length does not influence the choice between back-sighting and compass. Both work equally regardless of how long the traverse is. |
| B — Speed over accuracy | This is the rationale for using a trough compass, not back-sighting. The compass is the quick method. Back-sighting is chosen for accuracy, not speed. |
| D — No second point available | If no second reference point is available, you cannot back-sight. In that case you must use the trough compass. Option D describes the condition for using a compass, not back-sighting. |
Orientation Methods — Comparison
| Method | Used when | Accuracy |
|---|---|---|
| Trough compass | First station, or when no plotted back-point is visible | Lower (magnetic error) |
| Back-sighting | Current station is already plotted; previous station is visible | Higher (geometric, no magnetic error) |
Key Concepts for Students
- Compass first, back-sight for the rest: In practice, the compass is used only at the very first instrument station to establish the initial north direction on the sheet. At every subsequent station (assuming it is already plotted), back-sighting gives a more reliable orientation because it avoids magnetic errors and local attraction.
- Back-sighting requires a plotted station: You cannot back-sight if the current station’s position is unknown. That is why resection (Lehmann’s or two-point method) must be used when setting up at an un-plotted point — back-sighting alone cannot fix an unknown station.
- Orientation error is cumulative: Each slight misalignment at one station propagates through the survey. Back-sighting geometrically corrects orientation at each setup and prevents cumulative angular drift, making it the preferred method whenever the conditions allow it.