Compass surveying fundamentals: What is the horizontal angle between the true meridian and the magnetic meridian called in geodetic and engineering surveying?

Introduction / Context:
In compass and theodolite work, directions may be referenced to the true (astronomical) meridian or to the magnetic meridian. The horizontal angle between these two is a fundamental correction that surveyors must understand and apply when converting magnetic bearings to true bearings for mapping and layout tasks.

Given Data / Assumptions:

• The true meridian is defined by a plane through the geographic North–South poles and the survey station.
• The magnetic meridian is defined by the direction of the Earth’s magnetic field at the station.
• We are measuring a horizontal angle at the station between these two reference directions.

Concept / Approach:
The angle between the true meridian and the magnetic meridian is called magnetic declination (also termed magnetic variation). By convention, declination is positive (easterly) when the magnetic north lies east of true north, and negative (westerly) when it lies west of true north. This quantity varies with place and time and is distinct from “dip,” which is a vertical inclination of the magnetic field, and from “convergence,” which relates to the angle between meridians over a map projection.

Step-by-Step Solution:

Verification / Alternative check:
Field practice converts magnetic bearings to true bearings using: True bearing = Magnetic bearing +/− declination, with proper sign for easterly or westerly declination. This confirms the role and name of the angle.

Why Other Options Are Wrong:

• Bearing: a direction of a line from a reference meridian, not the angle between meridians.
• Dip: the vertical inclination of Earth’s magnetic field, not a horizontal angle.
• Convergence: the angle between grid and true meridians or between meridians on a projection, not between true and magnetic meridians.

Common Pitfalls:
Confusing declination with dip; forgetting that declination changes over time and requires current local values; mixing up the sign convention for easterly/westerly declination.

Final Answer:
Magnetic declination