Plane Table Survey – When to Use the Two-Point Problem for Orientation Orientation of a plane table by solving the two-point problem is preferably adopted under which field condition?

Introduction:
In plane table surveying, orientation aligns the plotted map with the ground. There are multiple resection problems: one-, two-, and three-point. This question checks when the two-point problem is the practical choice compared with the more robust three-point resection.

Given Data / Assumptions:

• The station to be plotted cannot be occupied (inaccessible or unsafe).
• Only two previously plotted control points are visible.
• Plane table resection is to be used to determine the station position and orientation.

Concept / Approach:

The three-point problem is generally superior in accuracy and determinacy. However, if only two well-separated control points are intervisible and the ground constraints prevent occupying the desired station, the two-point problem provides a workable orientation and location using auxiliary constructions (e.g., tracing paper or trial-and-error rotation about one point).

Step-by-Step Solution:

Verification / Alternative check:

Standard plane table procedures list the two-point problem as the method of choice when only two known points are usable and site constraints prevent normal occupation.

Why Other Options Are Wrong:

Two-point is not more accurate than three-point; it is chosen due to constraints, not simply to save time; many surrounding points would favour three-point resection or orientation by backsight.

Common Pitfalls:

Using two-point when three good points are available; choosing poorly separated control points leading to weak geometry.

Final Answer:

When the required station is inaccessible and only two known points can be sighted