Q13. Which of the following errors is most likely to occur in plane table surveying?
📚 Detailed Explanation: Why Orientation Error is the Most Likely in Plane Table Surveying
All four options list genuine sources of error in plane table surveying. The question asks which is most likely to occur. This requires understanding the mechanics of each error type and how often each arises in practice.
Orientation must be re-established correctly at every new instrument station. It involves rotating the table so that the lines on the sheet remain parallel to the corresponding ground lines — a geometric operation that is sensitive to even slight angular misalignment. Because the operation is repeated at every station, each setup is an opportunity for cumulative orientation error. Even a very small angular mistake in orientation causes all rays drawn from that station to be systematically off-direction, introducing structural errors across the entire map.
Comparing All Four Error Types
| Error type | How it arises | Frequency / likelihood |
|---|---|---|
| B — Orientation | Table rotated even slightly off-direction at each new station; cumulative through traverse | Highest — repeated at every station; sensitive to minor angular shifts from wind, ground vibration, or careless back-sighting |
| A — Sighting | Parallax between alidade vane and plotted point; observer’s eye not correctly aligned | Moderate — can be minimised by careful observation; alidade vane position reduces gross errors |
| C — Levelling | Spirit bubble not exactly centred; table slightly tilted | Lower — levelling is done at each station but is easily checked with the spirit level; obvious if the bubble is off |
| D — Measurement | Incorrect reading or scaling of distances onto the ray | Lower — chaining/taping errors are systematic and generally small; scaling onto the sheet is straightforward |
Why Orientation Error Has Disproportionate Impact
An angular error in orientation affects every ray drawn from that station simultaneously — all plotted features from that setup are wrong by the same angular amount. In contrast, a sighting error typically affects only one specific ray, and a measurement error affects only one plotted distance. Orientation error therefore has the widest structural impact on the map, making it both the most common and the most consequential error in plane table surveying.
Key Concepts for Students
- Orientation is repeated at every station: In a traverse of ten stations, orientation must be correctly established ten times. Each is an independent opportunity for error. Sighting, levelling, and measurement are performed for individual observations, but orientation is a system-wide operation at each setup.
- Back-sighting minimises orientation error: Using a previously plotted direction for back-sighting rather than the trough compass provides a geometric check on orientation. A compass reading can be off by 1–2° due to local attraction; back-sighting eliminates this source of orientation error.
- Orientation error is the exam answer for “most likely”: This question type appears frequently. The standard textbook answer is always orientation error, because it is structural (affects all rays from a station), cumulative (builds up through the traverse), and the hardest to detect visually during field operations.