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Quadrantal/WCB; declination, isogonic, agonic

Short answer

Whole circle bearing is measured clockwise from north 0°–360°; quadrantal bearing is an acute angle from N or S towards E or W. Magnetic declination is the angle between true and magnetic meridians. An isogonic line connects points of equal declination; the agonic line is the isogonic line with zero declination, where magnetic and true north coincide.

What are quadrantal and whole circle bearings? Explain magnetic declination, isogonic line and agonic line.

Quadrantal and whole circle bearings

  • Whole circle bearing (WCB): angle of a line from the north end of the meridian, clockwise, 0∘0^\circ–360∘360^\circ (prismatic compass).
  • Quadrantal bearing (QB): acute angle from the nearest N or S end of the meridian towards E or W, written like N 35∘35^\circ E (surveyor's compass).
WCBQB
0∘0^\circ–90∘90^\circN θ\theta E
90∘90^\circ–180∘180^\circS (180∘−θ)(180^\circ - \theta) E
180∘180^\circ–270∘270^\circS (θ−180∘)(\theta - 180^\circ) W
270∘270^\circ–360∘360^\circN (360∘−θ)(360^\circ - \theta) W

Magnetic declination

The angle between the true meridian and the magnetic meridian at a place. East if the magnetic north is east of true north (positive), west if it is west (negative).

True bearing=Magnetic bearing±declination\text{True bearing} = \text{Magnetic bearing} \pm \text{declination}

Isogonic line

An imaginary line on a map (or the earth) joining places having the same magnetic declination.

Agonic line

The isogonic line along which the declination is zero, that is, where the magnetic and true meridians coincide. The magnetic bearing equals the true bearing on this line. Declination is east on one side of it and west on the other.

✓ Answer checked by Claude Opus 5.5 (high effort)

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