Q6. When only one point is available for orientation, orientation is done by:
📚 Detailed Explanation: Orientation Using a Trough Compass
Orientation is the process of rotating the plane table horizontally at each station so that all plotted lines on the drawing sheet remain parallel to the corresponding lines on the ground. The method chosen depends on what reference information is available in the field.
Why Option D is Correct
Back-sighting requires a previously plotted station that is also visible on the ground — that means at least two known geometric references (the current station on the sheet and a visible back-station). Intersection and fore-sighting also require multiple known points. When only one point is available (e.g., at the very first instrument station of a survey), the trough compass is the only viable option because it relies on the independent reference of magnetic north rather than on a geometric back-sight.
Why the Other Options Are Wrong
| Option | Why it fails with only one point |
|---|---|
| A — Fore-sighting | Fore-sighting draws a ray toward a future (un-plotted) target station. It establishes a direction but cannot orient the table because the reference direction is unknown until you set up at the next station. |
| B — Back-sighting | Back-sighting requires at least two plotted points: the current station and a visible previous station. With only one reference point, you cannot create a known geometric direction to back-sight along. |
| C — Intersection | Intersection is a method of plotting unknown features by sighting from two known instrument stations. It is not an orientation method and requires two instrument setups. |
Key Concepts for Students
- Compass = first station orientation: The trough compass is typically used only at the very first instrument station of a plane table survey, where no previously plotted back-station exists. At all subsequent stations, back-sighting gives superior accuracy and is preferred.
- Limitation of compass orientation: Compass orientation is subject to magnetic declination (difference between magnetic and true north) and local magnetic attraction from nearby metal objects or magnetic rock formations. This makes it less accurate than back-sighting.
- One point vs two points: A common exam pattern contrasts these scenarios: “one point available → trough compass” vs “station already plotted → back-sighting.” Recognise which scenario the question describes before answering.