Q25. Plotting of inaccessible points on a plane table is done by:
Correct Answer: B. intersection
📚 Detailed Explanation: Intersection Method for Inaccessible Points
An inaccessible point is one that can be clearly seen from the instrument stations but cannot be physically reached to measure a distance directly to it — examples include a cliff face, a distant hill peak, the far bank of a river, or an island. The intersection method solves this problem by using two instrument stations, both already plotted, to draw rays toward the target from different directions. Where the two rays cross on the sheet is the plotted position of the inaccessible point.
Fig: Two sighting rays from known stations A and B intersect at the plotted position of inaccessible point P. No physical measurement to P is needed.
Intersection procedure for inaccessible points:
1. Set up the plane table at known station A. Sight the inaccessible target P and draw a ray aP.
2. Move the table to known station B. Orient correctly by back-sighting to A.
3. Sight the same target P from B and draw ray bP.
4. The intersection of the two rays fixes the position of P on the sheet.
Only the baseline distance AB needs to be physically measured on the ground.
1. Set up the plane table at known station A. Sight the inaccessible target P and draw a ray aP.
2. Move the table to known station B. Orient correctly by back-sighting to A.
3. Sight the same target P from B and draw ray bP.
4. The intersection of the two rays fixes the position of P on the sheet.
Only the baseline distance AB needs to be physically measured on the ground.
Why the Other Options Are Wrong
| Option | Why it cannot plot inaccessible points |
|---|---|
| A — Radiation | Radiation requires measuring the ground distance to the target point after sighting it. If the target is inaccessible, the distance cannot be measured, so radiation fails. |
| C — Traversing | Traversing moves the instrument to successive stations. It can only plot stations the instrument physically occupies — it cannot plot a point the instrument cannot reach. |
| D — Resection | Resection locates the instrument station itself, not an external target. It is used when the surveyor stands at an unknown position; it cannot plot a remote inaccessible feature. |
Key Concepts for Students
- Intersection = inaccessible, intersection = no distance to target: The two key facts about the intersection method. No other plane table method can locate a point that cannot be physically reached for distance measurement.
- Good intersection geometry requires well-angled rays: For the most precise fix, rays from A and B should cross at angles between 30° and 150° (ideally near 90°). Very acute or very obtuse crossing angles produce a large, elongated intersection region, reducing positional accuracy.
- Three rays give a check: If a third instrument station C is available, a third ray can be drawn to P. If all three rays pass through the same point, the intersection fix is verified. A small triangle formed by three slightly misaligned rays indicates small measurement or orientation errors at the three stations.