Geomagnetism in surveying — what is the term for an imaginary line that passes through all points on the Earth having equal magnetic declination? Choose the correct term.

Introduction / Context:
Magnetic declination is the horizontal angle between magnetic north and true north. Surveyors must account for declination to relate compass bearings to true meridians. Several families of geomagnetic lines describe equal values of different magnetic parameters across the Earth’s surface.

Given Data / Assumptions:

• Declination varies with location and time.
• Terminology relates to maps of Earth’s magnetic field.
• We seek the term associated specifically with equal declination.

Concept / Approach:
An isogon (or isogonic line) connects points of equal magnetic declination. An agonic line is a special isogon where declination is zero (magnetic north equals true north). An isoclinic line connects points of equal magnetic dip (inclination), a different parameter from declination.

Step-by-Step Solution:

Verification / Alternative check:
Geomagnetic charts publish isogonic lines with values in degrees east or west; agonic line is labeled 0°.

Why Other Options Are Wrong:

• Agonic: Only where declination is zero, not a general equal-declination line.
• Isoclinic: Equal dip (inclination), not declination.
• None: Incorrect because “isogon” is the established term.

Common Pitfalls:
Confusing declination with inclination; they are orthogonal components of the geomagnetic field.

Final Answer:
Isogon (isogonic line)