Magnetic terms and lines: identify the correct statements about declination and related isogonic/agonic lines.
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AThe horizontal angle between the magnetic meridian and the true meridian at a place is called magnetic declination (variance of the compass).
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BImaginary lines joining points of equal magnetic declination at a given time are called isogonic lines.
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CIsogonic lines through places where declination is zero are called agonic lines.
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DAll the above.
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ENone of the above
Answer
Correct Answer: All the above.
Explanation
Introduction / Context:Converting between magnetic and true bearings requires understanding magnetic declination and the map of its spatial variation. Standard terms include declination, isogonic lines, and agonic lines, used on navigational and topographic charts.
Given Data / Assumptions:
- Magnetic meridian differs from the true meridian by declination.
- Declination varies with place and time.
- Charts show contours of equal declination.
Concept / Approach:
Magnetic declination is the angle between magnetic and true meridians (east or west). Lines of equal declination are isogonic lines; the special case where declination is zero (magnetic and true meridians coincide) defines agonic lines.
Step-by-Step Solution:
Define declination = angle(Magnetic meridian, True meridian).Isogonic = loci of equal declination values.Agonic = the isogonic with value zero.Verification / Alternative check:
Map margins and geophysical references adopt these definitions universally.
Why Other Options Are Wrong:
Each A–C is correct; thus D (“All the above”) must be selected; E is false.
Common Pitfalls:
Confusing agonic with isoclinic (equal dip); mixing grid convergence with declination.
Final Answer:
All the above.