Time variation of magnetic bearings – behavior of a survey line's magnetic direction How does the magnetic bearing of a survey line at a given place behave over time?

Introduction / Context:
Magnetic bearings are measured with respect to the local magnetic meridian, which is influenced by Earth’s magnetic field. This field changes with time and space, so magnetic bearings are not immutable. Surveyors must understand the nature of these changes to apply appropriate corrections and maintain consistency over projects conducted at different times.

Given Data / Assumptions:

• Magnetic declination varies geographically and temporally.
• Both secular (long-term) and short-term (annual, diurnal, magnetic storms) variations occur.
• True (astronomic/geodetic) bearings are assumed constant for practical time scales.

Concept / Approach:

The magnetic bearing of a line equals the true bearing plus (or minus) the local magnetic declination. Because declination changes slowly over years (secular variation) and exhibits smaller periodic variations, the magnetic bearing changes systematically with time. It does not remain constant, nor does it necessarily change only month-to-month; and it is not always greater than the true bearing—its relation depends on the sign of declination at the location and epoch.

Step-by-Step Solution:

Verification / Alternative check:

Magnetic observatory data and isogonic charts document predictable long-term drifts and smaller periodic components, demonstrating that bearings referenced to magnetic north do not remain fixed over time.

Why Other Options Are Wrong:

Option A ignores well-known temporal variation.

Option C narrows variation incorrectly to months only; changes are multi-scale.

Option D is false because MB can be greater or smaller than TB depending on declination sign.

Common Pitfalls:

Re-using old magnetic bearings without epoch correction; assuming negligible change over project durations when working near regions of rapid secular variation.

Final Answer:

It changes systematically due to magnetic variation and secular change