Latitude and Longitude (Meridians and Parallels)
The equator is an imaginary circle equidistant from the poles of the Earth. Circles parallel to the equator (lines running east and west) are parallels of latitude. They are used to measure degrees of latitude north (N) or south
(S) of the equator. The angular distance from the equator to the pole is one-fourth of a circle or 90°. The 48 conterminous states of the United States are located between 25° and 49° N latitude. The arrows in Figure 11-3 labeled “Latitude” point to lines of latitude. Meridians of longitude are drawn from the North Pole to the South Pole and are at right angles to the Equator. The “Prime Meridian,” which passes through Greenwich, England, is used as the zero line from which measurements are made in degrees east (E) and west
(W) to 180°. The 48 conterminous states of the United
Figure 11-3. Meridians and parallels—the basis of measuring time, distance, and direction.
States are between 67° and 125° W longitude. The arrows in Figure 11-3 labeled “Longitude” point to lines of longitude.
Any specific geographical point can be located by reference to its longitude and latitude. Washington, D.C., for example, is approximately 39° N latitude, 77° W longitude. Chicago is approximately 42° N latitude, 88° W longitude.
Variation
Variation is the angle between true north (TN) and magnetic north (MN). It is expressed as east variation or west variation depending upon whether MN is to the east or west of 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 11-4. If the Earth were uniformly magnetized, the compass needle would point toward the magnetic pole, in which case the variation between TN (as shown by the geographical meridians) and MN (as shown by the magnetic meridians) could be measured at any intersection of the meridians.
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 that connect points of equal magnetic variation. (The line connecting points at which there is no variation between TN and MN is the agonic line.) An isogonic chart is shown in Figure 11-5. Minor bends and turns in the isogonic and agonic lines are caused by unusual geological conditions affecting magnetic forces in these areas.
Antenna Towers
Extreme caution should be exercised when flying less than 2,000 feet AGL because of numerous skeletal structures, such as radio and television antenna towers, that exceed 1,000 feet AGL with some
Figure 11-4. Magnetic meridians are in red while the lines of longitude and latitude are in blue. From these lines of variation (magnetic meridians), on can determine the effect of local magnetic variations on a magnetic compass.
Figure 11-5. Note the agonic line where magnetic variation is zero.
extending higher than 2,000 feet AGL. Most skeletal structures are supported by guy wires which are very difficult to see in good weather and can be invisible at dusk or during periods of reduced visibility. These wires can extend about 1,500 feet horizontally from a structure; therefore, all skeletal structures should be avoided horizontally by at least 2,000 feet.
Additionally, new towers may not be on your current chart because the information was not received prior to the printing of the chart.