Survey reference directions — why are true meridians generally preferred over magnetic meridians for establishing bearings and control?

Introduction / Context:
Bearings are referenced either to the true (geographic) meridian or to the magnetic meridian. The choice affects long-term consistency of surveys, mapping, and the integration of different datasets. Understanding the advantages of using the true meridian is critical for geodetic control and cadastral work.

Given Data / Assumptions:

• True meridian is the line through a point toward the geographic North Pole.
• Magnetic meridian aligns with the Earth’s magnetic field at a point and time.
• Survey networks may be used and compared over many years.

Concept / Approach:

The principal advantage of the true meridian is its long-term constancy for a given location (aside from small, well-modeled geodetic effects). Magnetic meridians suffer from secular variation, diurnal changes, magnetic storms, and local attraction (e.g., iron structures, rocks), causing bearings to drift with time or space. Using true meridians ensures that bearings remain comparable across epochs and independent of transient magnetic anomalies.

Step-by-Step Solution:

Verification / Alternative check:

National mapping agencies publish geodetic coordinates and grid systems tied to true/geodetic north, not to the magnetic meridian, reflecting this preference.

Why Other Options Are Wrong:

Converge to a point — true for all meridians at the poles but not the reason for preference.
Change due to time — describes magnetic meridian behaviour; it is a disadvantage, not a reason for preference.
None of these — incorrect because one option clearly states the correct rationale.

Common Pitfalls:

Assuming “grid north” and “true north” are identical everywhere; grid convergence may need correction, yet both are more stable than magnetic north for control.

Final Answer:

these remain constant.