Q20. The intersection method in plane table surveying is most suitable for:
Correct Answer: C. Hilly areas
📚 Detailed Explanation: Why Intersection is Best Suited for Hilly Areas
The intersection method locates field features by drawing sighting rays from two known, plotted instrument stations. The intersection of the two rays on the sheet gives the position of the target point, without any distance measurement to the target. This makes it the ideal method when physical access to the target is impossible or impractical.
Fig: Intersection method. Only the baseline AB is measured on the ground. The inaccessible hill peak P is fixed on the sheet by the intersection of sighting rays from A and B — no physical measurement to P is needed.
Why hilly areas are ideal for intersection:
In hilly terrain, many key features (mountain peaks, cliff edges, far ridgelines) are physically inaccessible or too dangerous to approach for direct distance measurement. The intersection method requires only that the target be visible from two known stations — no physical access is needed. Only the baseline distance AB between the two instrument stations needs to be measured on the ground.
In hilly terrain, many key features (mountain peaks, cliff edges, far ridgelines) are physically inaccessible or too dangerous to approach for direct distance measurement. The intersection method requires only that the target be visible from two known stations — no physical access is needed. Only the baseline distance AB between the two instrument stations needs to be measured on the ground.
Why the Other Options Are Less Suitable
| Option | Why intersection is less suited here |
|---|---|
| A — Forests | In dense forest, sighting lines are obstructed by trees. The intersection method requires clear lines of sight from two separate stations to the same target. Forests block these sightings, making the method impractical. |
| B — Urban areas | In urban settings, buildings are accessible; distances can be measured directly. The radiation method is more efficient and equally accurate for plotting accessible urban details. |
| D — Plains | In flat, open terrain all points are accessible, so direct measurement by radiation or traversing is simpler and faster. Intersection’s advantage (handling inaccessibility) is not needed on a flat plain. |
Key Concepts for Students
- Intersection = inaccessible targets: The defining feature of the intersection method is that only a baseline needs to be measured; the target itself need not be reached. Whenever a question mentions cliff edges, hill peaks, far riverbanks, or any inaccessible point, intersection is the method to select.
- Good intersection angles matter: The intersection rays from A and B should cross at an angle as close to 90° as possible. Rays crossing at very acute or very obtuse angles produce an elongated error ellipse and reduce precision. In hilly terrain, choosing the baseline position to achieve good intersection geometry is an important field skill.
- Intersection vs. resection: Both use rays from multiple known points, but the goal differs. Intersection fixes an external target; resection fixes the instrument station itself. In hilly areas, intersection is used to plot hilltops; resection is used to establish instrument positions on inaccessible ridges.