Astronomical surveying – meridians: The true meridian at a place is the intersection on the Earth’s surface of a plane passing through which reference line?

Introduction / Context:
Meridians are essential reference lines for azimuths, bearings, and map orientation. Surveyors distinguish between the astronomical/true meridian and the magnetic meridian. The question tests the definition of the true meridian used in precise work and for control networks.

Given Data / Assumptions:

• The Earth has geographic (true) poles defined by its axis of rotation.
• Magnetic poles differ in position and shift slowly over time.
• Meridian planes intersect the Earth’s surface along meridian lines.

Concept / Approach:
The true meridian is the line of intersection of the Earth’s surface with the plane that passes through the geographic North–South poles and the observation station. This line is independent of the magnetic field and is fundamental for astronomical azimuths and triangulation baselines.

Step-by-Step Solution:

Verification / Alternative check:
True meridian direction can be established by solar observations (e.g., equal altitude method) or stellar transits, all referencing the Earth’s rotational axis, not the magnetic field.

Why Other Options Are Wrong:

• East–West points and zenith–nadir define other great circles/vertical planes, not the true meridian plane.
• Magnetic poles define the magnetic meridian, which differs from the true meridian by the declination angle.

Common Pitfalls:
Confusing true and magnetic bearings; neglecting local magnetic disturbances; assuming fixed relation between true and magnetic meridians without applying current declination.

Final Answer:
The geographic (true) North and South poles