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Pilot's Handbook of Aeronautical Knowledge

Latitude and Longitude (Meridians and

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Compass rose.
Figure 15-6. Compass rose.

Because meridians converge toward the poles, course
measurement should be taken at a meridian near the midpoint
of the course rather than at the point of departure. The course
measured on the chart is known as the true course (TC). This
is the direction measured by reference to a meridian or true
north. It is the direction of intended flight as measured in
degrees clockwise from true north.

As shown in Figure 15-7, the direction from A to B would
be a true course of 065°, whereas the return trip (called the
reciprocal) would be a true course of 245°.

Courses are determined by reference to meridians.
Figure 15-7. Courses are determined by reference to meridians on
aeronautical charts.

The true heading (TH) is the direction in which the nose of
the aircraft points during a flight when measured in degrees
clockwise from true north. Usually, it is necessary to head
the aircraft in a direction slightly different from the true
course to offset the effect of wind. Consequently, numerical
value of the true heading may not correspond with that of
the true course. This is discussed more fully in subsequent
sections in this chapter. For the purpose of this discussion,
assume a no-wind condition exists under which heading and
course would coincide. Thus, for a true course of 065°, the
true heading would be 065°. To use the compass accurately,
however, corrections must be made for magnetic variation
and compass deviation.

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.

magnetic 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.

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.