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Latitude and Longitude (Meridians and Parallels) – Variation

by Flight Learnings

in Navigation

Variation is the angle between true north and magnetic north. It is expressed as east variation or west variation depending upon whether magnetic north (MN) is to the east or west of true north (TN).

The north magnetic pole is located close to 71° N latitude, 96° W longitude and is about 1,300 miles from the geographic or true north pole, as indicated in Figure 15-8. If the Earth were uniformly magnetized, the compass needle would point toward the magnetic pole, in which case the variation between true north (as shown by the geographical meridians) and magnetic north (as shown by the magnetic meridians) could be measured at any intersection of the meridians.

Figure 15-8. Magnetic meridians are in red while the lines of longitude and latitude are in blue. From these lines of variation (magnetic meridians), one can determine the effect of local magnetic variations on a magnetic compass.

Figure 15-8. Magnetic meridians are in red while the lines of longitude and latitude are in blue. From these lines of variation (magnetic meridians), one can determine the effect of local magnetic variations on a magnetic compass.

Actually, the Earth is not uniformly magnetized. In the United States, the needle usually points in the general direction of the magnetic pole, but it may vary in certain geographical localities by many degrees. Consequently, the exact amount of variation at thousands of selected locations in the United States has been carefully determined. The amount and the direction of variation, which change slightly from time to time, are shown on most aeronautical charts as broken magenta lines, called isogonic lines, which connect points of equal magnetic variation. (The line connecting points at which there is no variation between true north and magnetic north is the agonic line.) An isogonic chart is shown in Figure 15-9. Minor bends and turns in the isogonic and agonic lines are caused by unusual geological conditions affecting magnetic forces in these areas.

Figure 15-9. Note the agonic line where magnetic variation is zero.

Figure 15-9. Note the agonic line where magnetic variation is zero.

On the west coast of the United States, the compass needle points to the east of true north; on the east coast, the compass needle points to the west of true north.

Zero degree variation exists on the agonic line, where magnetic north and true north coincide. This line runs roughly west of the Great Lakes, south through Wisconsin, Illinois, western Tennessee, and along the border of Mississippi and Alabama. [Compare Figures 15-9 and 15-10.]

Figure 15-10. Effect of variation on the compass.

Figure 15-10. Effect of variation on the compass.

Because courses are measured in reference to geographical meridians which point toward true north, and these courses are maintained by reference to the compass which points along a magnetic meridian in the general direction of magnetic north, the true direction must be converted into magnetic direction for the purpose of flight. This conversion is made by adding or subtracting the variation which is indicated by the nearest isogonic line on the chart.

For example, a line drawn between two points on a chart is called a true course as it is measured from true north. However, flying this course off the magnetic compass would not provide an accurate course between the two points due to three elements that must be considered. The first is magnetic variation, the second is compass deviation, and the third is wind correction. All three must be considered for accurate navigation.

Magnetic Variation

As mentioned in the paragraph discussing variation, the appropriate variation for the geographical location of the flight must be considered and added or subtracted as appropriate. If flying across an area where the variation changes, then the values must be applied along the route of flight appropriately. Once applied, this new course is called the magnetic course.

Magnetic Deviation

Because each aircraft has its own internal effect upon the onboard compass systems from its own localized magnetic influencers, the pilot must add or subtract these influencers based upon the direction he or she is flying. The application of deviation (taken from a compass deviation card) compensates the magnetic course unique to that aircraft’s compass system (as affected by localized magnetic influencers) and it now becomes the compass course. Therefore, the compass course when followed (in a no wind condition) takes the aircraft from point A to point B even though the aircraft heading may not match the original course line drawn on the chart.

If the variation is shown as “9° E,” this means that magnetic north is 9° east of true north. If a true course of 360° is to be flown, 9° must be subtracted from 360°, which results in a magnetic heading of 351°. To fly east, a magnetic course of 081° (090° – 9°) would be flown. To fly south, the magnetic course would be 171° (180° – 9°). To fly west, it would be 261° (270° – 9°). To fly a true heading of 060°, a magnetic course of 051° (060° – 9°) would be flown.

Remember, if variation is west, add; if east, subtract. One method for remembering whether to add or subtract variation is the phrase “east is least (subtract) and west is best (add).”

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