Q19. Detailed plotting in plane table surveying is generally done by:
Correct Answer: B. radiation
📚 Detailed Explanation: Radiation Method for Detailed Plotting
In plane table surveying, different methods serve different purposes. For the task of detailed plotting — locating and drawing the many individual features of the terrain (buildings, roads, fences, trees, water bodies) — the radiation method is the standard technique used.
Fig: Radiation method. The alidade is pivoted around known station S to sight features P1–P6. A ray is drawn for each feature; the measured ground distance is scaled onto the ray to fix the feature’s position on the sheet.
How radiation works:
1. Set up the plane table at the known station S (already plotted on sheet).
2. Place the alidade with its fiducial edge through point s on the sheet.
3. Rotate the alidade to sight a field feature (P1, P2, etc.).
4. Draw a ray along the fiducial edge from s.
5. Measure the ground distance SP using a tape or tacheometric readings.
6. Scale the distance onto the ray to plot the feature’s position.
7. Repeat for all features visible from S.
1. Set up the plane table at the known station S (already plotted on sheet).
2. Place the alidade with its fiducial edge through point s on the sheet.
3. Rotate the alidade to sight a field feature (P1, P2, etc.).
4. Draw a ray along the fiducial edge from s.
5. Measure the ground distance SP using a tape or tacheometric readings.
6. Scale the distance onto the ray to plot the feature’s position.
7. Repeat for all features visible from S.
Why the Other Options Are Wrong
| Option | What it actually does | Why not used for detail plotting |
|---|---|---|
| A — Traversing | Moves between successive instrument stations through the area | Traversing establishes instrument stations, not individual field details. Detail is plotted by radiation from each traverse station. |
| C — Resection | Determines the instrument station’s own position | Resection cannot plot external field features — it only fixes where the table is standing. Once the station is fixed, radiation is used for detail. |
| D — Both (b) and (c) | — | Resection does not plot detail; only radiation does. “Both” is therefore incorrect. |
Key Concepts for Students
- Radiation = detail plotting from one station: Whenever a question asks about plotting surrounding field features (roads, buildings, topographic details) from a single known station, the answer is always radiation. It is the most efficient method for detail survey work.
- Radiation requires the station to be already plotted: Unlike resection (which fixes an unknown station), radiation requires the instrument position to be already established on the sheet. In a traverse, each traverse station is first fixed, then detail is radiated from it.
- Distance measurement method affects speed: In simple radiation, distances are measured by tape. In tachometric plane table surveying, the alidade is fitted with stadia hairs that allow simultaneous angle and distance determination, making radiation faster for topographic surveys with many detail points.