Direction, Latitude and Longitude — Page 8, Lesson 14

Let’s start with a key modern standard. For these reasons, ICAO has adopted WGS 84 as the world standard. WGS 84 stands for World Geodetic System 1984 — it’s a global reference frame that defines positions on Earth consistently for all aviation users. In modern navigation systems, position information is corrected for the distortions of the Earth’s shape automatically in the navigation computers. So you don’t have to manually adjust for the fact that the Earth isn’t a perfect sphere — the computer handles that. For any calculations that you may be required to do for the EASA examination syllabus, the Earth may be considered to be a true sphere with a circumference of 21 600 NM or 40 000 km. That’s a simplification for exam purposes: we treat the Earth as a perfect sphere, and we use those two circumference values — 21,600 nautical miles, or 40,000 kilometres — as the standard. Now, let’s talk about the Poles. The Poles are defined as the extremities of the axis about which the Earth spins. So the North Pole and South Pole are the two ends of the Earth’s rotation axis. The axis of the Poles is inclined to the axis of the Earth’s orbit around the Sun at an angle of 23½°. That tilt is important for seasons and time, and this topic will be covered more fully in the chapter on “Time”. However, in this chapter the polar axis will be drawn upright — we ignore the tilt for now. Next, basic direction on the Earth. To start to define directions on the Earth, a datum must be selected. A datum is a reference point or line we use as a starting place. The simplest datum is the direction in which the Earth is spinning. This is then defined as East, hence sunrise in the East. So because the Earth rotates from west to east, we call that direction East — that’s why the Sun appears to rise in the East. West is then defined as the opposite of East. Facing East, the pole on the left is called the North Pole and the direction North is defined as the direction towards the North Pole. So if you stand facing East, your left side points toward the North Pole — that’s North. The pole diametrically opposite the North Pole is called the South Pole and the direction South is defined as being opposite to North. So South is directly opposite North. These directions, North, South, East and West are known as Cardinal Points. That’s the formal term — the four cardinal points. The midway directions between North and East, East and South, South and West, and West and North are North-East (NE), South-East (SE), South-West (SW), and North-West (NW). These directions are known as the Quadrantal directions. So quadrantal means halfway between the cardinal points, giving you four more directions. To solve navigation problems, the student may occasionally need to consider the Earth when viewed from above either the North Pole or the South Pole. When viewed from above the North Pole, the Earth appears to rotate in an anticlockwise (counter-clockwise) direction. When viewed from above the South Pole, the Earth appears to rotate in a clockwise direction. So from the North Pole looking down, the spin is anticlockwise; from the South Pole looking up, it’s clockwise. That’s a useful mental picture for understanding wind patterns and some navigation concepts later on.