Q9. The principle of plane table survey is:
Correct Answer: C. Parallelism
📚 Detailed Explanation: The Principle of Parallelism in Plane Table Surveying
Every surveying method is built on a fundamental geometric principle. For plane table surveying, that principle is parallelism: at every instrument station, all rays drawn on the map sheet must be perfectly parallel to the corresponding lines on the ground.
Parallelism defined: If AB is a line on the ground connecting two survey points, then the corresponding plotted line ab on the drawing sheet must always be parallel to AB, regardless of which station the table is set up at. This parallelism is maintained by correctly orienting the table at each setup, using either back-sighting or the trough compass.
Why Parallelism is the Core Principle
When the table is oriented correctly, every sight taken through the alidade produces a ray on paper that is parallel to the actual ground line it represents. This allows the surveyor to plot distances along rays by simply scaling the measured ground distance onto the sheet — the geometry is preserved by the parallel relationship between sheet and ground. If orientation fails, the parallelism breaks down and the entire plotted network distorts.
Why the Other Options Are Wrong
| Option | What it actually is | Why it doesn’t define plane table |
|---|---|---|
| A — Traversing | Moving between successive stations along a route, measuring angles/distances at each | Traversing is one method used with a plane table (among others like radiation and intersection), not the underlying principle of the instrument itself. |
| B — Ranging | The process of aligning intermediate points on a straight survey line using ranging rods | Ranging is a chain-survey operation, unrelated to the principle of plane table surveying. |
| D — Triangulation | Establishing a control network by measuring angles of triangles from a known baseline | Triangulation is a high-precision control survey technique. While a plane table can perform intersection (a graphical form of triangulation), triangulation is not the governing principle of the plane table. |
Key Concepts for Students
- Parallelism requires correct orientation: The table must be oriented at each new station so the sheet aligns with the ground network. If the table is rotated even slightly off-orientation, rays drawn from that setup will not be parallel to the ground lines, corrupting the map.
- Orientation maintains parallelism station-to-station: As the survey progresses from station to station, back-sighting along previously plotted lines re-establishes the correct parallel relationship. This is why back-sighting is such an important technique — it enforces the parallelism principle at every setup.
- Distinguish principle from method: Questions sometimes ask for the “principle” (parallelism) separately from the “methods” (radiation, intersection, resection, traversing). The principle is what makes the instrument work; the methods are how it is applied. Always read the question carefully to see which is being asked for.