Plane-table orientation with two visible but inaccessible points: At a new station where two known points are visible but cannot be occupied or measured to, which method is generally adopted to orient the plane table?

Introduction / Context:Orienting a plane table means aligning the drawn map with the actual ground directions at a new station. When the reference points are known on the map but are inaccessible on the ground, we rely on geometrical resection techniques rather than occupying those points.

Given Data / Assumptions:

• Two known control points appear on the existing map.
• Both points are visible from the new station but cannot be occupied.
• Distances are not taped; the table must be oriented from sightings.

Concept / Approach:With two visible known points, the adopted procedure is the two-point problem (a special case of resection). By sighting those two points and using the geometry of their plotted positions, the plane table can be oriented so that lines of sight coincide with their map radiations. Intersection and radiation are for plotting new points; general three-point resection needs three references, while the two-point method works with two when auxiliary steps (like using a nearby point or approximate orientation) are employed.

Step-by-Step Solution:

Verification / Alternative check:Check by sighting a third well-identified point (if available) to confirm orientation.

Why Other Options Are Wrong:

• Intersection: used to fix an unknown point by crossing rays from two stations, not to orient the board.
• Resection (general) is correct in concept but non-specific; the question focuses on the two-point variant.
• Radiation: used to plot details from a single station.

Common Pitfalls:Failing to get a good approximate orientation before fine adjustment; misidentifying the mapped points on the ground.

Final Answer:Two-point problem (two-point resection)