When Magnetic direction is the same as True direction the variation is nil; otherwise Magnetic North direction may lie either to the West or East of the True North Direction. See Figures 3.16a
& 3.16b.
Figure 3.16 a. Westerly Variation Figure 3.16 b. Easterly Variation Figure 3.16a Westerly Variation Figure 3.16b Easterly Variation The direction of the aircraft The direction of the aircraft relative to True North is 105° relative to True North is 105°
The direction of the aircraft The direction of the aircraft relative to Magnetic North relative to Magnetic North
is 122°. is 088°.
Magnetic North is to the West Magnetic North is to the East
of True North of True North.
VARIATION IS 17° W VARIATION IS 17° E
The direction in which the fore and aft axis of the aircraft is pointing is known as the aircraft’s heading.
We are thus able to complete the following
tables:-From Figure 3.16a From Figure 3.16b
Deviation is the angle between Magnetic North and Compass North measured in degrees East or West from Magnetic North. Figures 3.17a & 3.17b illustrate this.
Figure 3.17a. Westerly DeviationFigure 3.17a Westerly Deviation Figure 3.17b. Easterly DeviationFigure 3.17b Easterly Deviation The direction of the aircraft The direction of the aircraft relative to True North is 100°. relative to Magnetic North is 100°
The direction of the aircraft The direction of the aircraft relative to Magnetic North is 125° relative to Magnetic North is 125°.
The direction of the aircraft The direction of the aircraft indicated by the Compass indicated by the Compass
is 135°. is 115°.
and we are able to complete the following tables:-From Figure 3.17a
and from this we deduce the following rule which must be
remembered:-The signs for plus and minus are sometimes used as a prefix instead of the suffixes East or West, in which case you will see that the rule is to apply the deviation (as stated. to the Compass Heading to obtain the Magnetic Heading and conversely apply the deviation (with the sign changed. to Magnetic Heading to obtain Compass Heading.
For example:-Deviation 3°E
Hdg 263(M) deviation 3°E Deviation East Compass Least Hdg 260(C) Hdg 260(C) deviation 3°E Deviation East Compass Least Hdg 263(M) Alternatively Deviation +3°
Hdg 263(M) deviation +3 to convert from Magnetic to Compass.
Change the sign and apply to Hdg(M);
Hdg 263 - 3 = Hdg 260(C) Or
Hdg 260(C) + 3 to convert from Compass to Magnetic.
Apply deviation to Hdg(C) Hdg 263 - 3 = Hdg 260(C)
Another rule, equivalent to the one stated above, is also used:-From Compass to True the signs are true (E is +, W is -)
DEFINITIONS
Definitions of terms associated with direction on the Earth.
Heading is the direction in which the fore and aft axis of the aircraft is pointing; it may be measured from True, Magnetic, or Compass North.
Variation is the angle measured at a point between True North direction and the direction indicated by a freely suspended compass needle influenced only by the Earth’s magnetic field.
It is termed East or West according to whether Magnetic North lies to the East or West of True North.
Deviation is the angle measured at a point between the direction indicated by a compass needle and the direction of Magnetic North. It is termed East or West according to whether the Compass North lies to the East or West of Magnetic North.
Isogonals are pecked lines on a map or chart joining places of equal magnetic variation.
Agonic Line is the name given to isogonals joining places of zero variation.
The Angle of Dip is the angle in the vertical plane between the horizontal and the Earth’s magnetic field at a point.
Isoclinals are lines on a map or chart joining places of equal magnetic dip.
Aclinic Lines is the name given to isoclinals joining places of zero dip.
Note:- Isoclinals and Aclinic lines do not appear on Navigation charts.
QUESTIONS
1. The sensitivity of a direct reading magnetic compass is:
a. Inversely proportional to the horizontal component of the earth’s magnetic field.
b. Proportional to the horizontal component of the earth’s magnetic field.
c. Inversely proportional to the vertical component of the earth’s magnetic field.
d. Inversely proportional to the vertical and horizontal components of the earth’s magnetic field.
2. What is the definition of magnetic variation?
a. The angle between the direction indicated by a compass and Magnetic North.
b. The angle between True North and Compass North.
c. The angle between Magnetic North and True North.
d. The angle between Magnetic Heading and Magnetic North.
3. At the magnetic equator:
a. Dip is zero b. Variation is zero c. Deviation is ero
d. The isogonal is an agonic line
4. Which of these is a correct statement about the Earth’s magnetic field?
a. It acts as though there is a large blue magnetic pole in Northern Canada b. The angle of dip is the angle between the vertical and the total magnetic force.
c. It may be temporary, transient, or permanent.
d. It has no effect on aircraft deviation.
5. Where is a compass most effective?
a. About midway between the earth’s magnetic poles b. In the region of the magnetic South pole
c. In the region of the magnetic North pole d. On the geographic equator
6. The value of variation:
a. is zero at the magnetic equator b. has a maximum value of 180°
c. has a maximum value of 45° E or 45° W d. cannot exceed 90°
7. The agonic line:
a. is midway between the magnetic North and South poles b. follows the geographic equator
c. is the shorter distance between the respective True and Magnetic North and South
8. The angle between True North and Magnetic north is known as:
a. deviation b. variation c. alignment error
d. dip
9. The value of magnetic variation on a chart changes with time. This is due to:
a. Movement of the magnetic poles, causing an increase b. Increase in the magnetic field, causing an increase c. Reduction in the magnetic field, causing a decrease
d. Movement of the magnetic poles, which can cause either an increase or a decrease 10. Isogonal lines converge as follows:
a. At the North Magnetic Pole
b. At the North and South Magnetic and Geographical Poles c. At the North and South Magnetic Poles
d. At the Magnetic equator.
11. What is the maximum possible value of Dip Angle?
a. 66°
b. 180°
c. 90°
d. 45°
12. What is the dip angle at the South Magnetic Pole?
a. 0°
b. 90°
c. 180°
d. 64°
13. What is a line of equal magnetic variation?
a. An isocline b. Compass North is West of Magnetic North c. True North is East of Magnetic North d. Magnetic North is West of Compass North
15. Fill in the blank spaces in the table below.
°TRUE VARN °MAG DEVN °COMP
260 291 3E
10W 1E 070
7W 001 2E
17W 0 020
306 10E 1W
036 031 033
5E 025 023
359 3W 2E
23E 2W 221
312 322 319
002 3W 1W
260 291 -3
5E +1 070
3W 001 +2
022 10W 3W 035
ANSWERS
1 B 6 B 11 C
2 C 7 D 12 B
3 A 8 B 13 B
4 A 9 D 14 C
5 A 10 B
15.
°TRUE VARN °MAG DEVN °COMP
260 31W 291 3E 288
061 10W 071 1E 070
354 7W 001 2E 359
003 17W 020 0 020
306 10E 296 1W 297
036 5E 031 2W 033
030 5E 025 2E 023
359 3W 002 2E 000
242 23E 219 2W 221
312 10W 322 3E 319
002 3W 005 1W 006
260 31W 291 -3 294