Plane table orientation – Bessel’s method: For orienting a plane table by three well-defined points A, B, and C, which description best states Bessel’s drill (sequence of sights and rays)?

Introduction / Context:
When re-orienting a plane table without resetting on the same station mark, Bessel’s method (drill) provides a practical sequence of sights to ensure the table is correctly oriented using three known points plotted on the sheet. It is widely taught for reliability and simplicity in the field.

Given Data / Assumptions:

• Three well-defined, intervisible points A, B, C are plotted accurately on the table.
• The alidade can be used to draw rays from plotted points toward others.
• The goal is to bring the table into the correct orientation so that plotted lines align with ground directions.

Concept / Approach:
Bessel’s drill uses two successive orientations and ray drawings that converge to a unique intersection on the sheet. By aligning B through A and drawing a ray toward C, then aligning A through B and drawing another ray toward C, the intersection of the two rays marks the proper plotted position through which C should be sighted to complete the orientation accurately.

Step-by-Step Solution:

Verification / Alternative check:
Field texts present equivalent step orders as long as the logic of intersecting rays to the third point is preserved; option A states the classical sequence verbatim.

Why Other Options Are Wrong:

• Options B and C permute roles incorrectly relative to the classical description.
• Option D is vague and does not specify the canonical order; Bessel’s drill is prescriptive rather than arbitrary.

Common Pitfalls:
Drawing rays from wrong plotted points; parallax or poor sighting; not clamping the table between steps, leading to misalignment.

Final Answer:
Align B through A and draw a ray toward C; align A through B and draw a ray toward C; finally, align C through the intersection of the two rays.