Direction, Latitude and Longitude 1 — Page 8, Lesson 16

I want to walk you through how we define direction in air navigation, starting with the older systems and then moving into the precise system we use today. The older maritime navigation system used Cardinal and Quadrantal directions — North, South, East, West, and subdivisions like Northeast, Southwest, and so on. Mariners subdivided those further to get more precision, but the system became too cumbersome. It simply does not allow the precise definition of direction that air navigation demands. So, while we retain the cardinal points as a reference, we use the Sexagesimal system for measuring directions. Let me define that term precisely. The Sexagesimal system is based on the fact that a clockwise rotation of direction from North, through East, South, and West, and back to North, forms a complete circle of 360 degrees. If we define North as 000°, then East becomes 090°, South becomes 180°, and West becomes 270°. Continuing the rotation back to North makes North 360°. In practice, using 000° or 360° for North is a personal choice — though a northerly runway would be labelled '36', which we will come back to. Now, when we measure direction on the Earth in degrees clockwise from North, and the North datum is the direction of the North geographic pole, that is called True direction. We use the abbreviation (T). So, North is referred to as 000°(T), East as 090°(T), South as 180°(T), and West as 270°(T). A critical point: we always use a three-figure group. It is 000°(T), not 0°(T), and 090°(T), not 90°(T). The reason is to avoid ambiguity, especially when transmitting messages by radio telephony, or R/T, or by any other means. If you see a direction of less than three figures, treat it as suspect. For example, 27° could be a corruption of 027°, 270°, 127°, 227°, 327°, 271°, 272°, and so on. There is one exception to this rule: runway directions are given at 10° intervals, to the nearest 10°. So a runway orientated at 273° is called Runway 27, and a runway orientated at 078° becomes Runway 08. But beware — runway directions are normally given with reference to Magnetic North, not True North. We will cover magnetic direction later. Finally, we need to talk about reciprocal directions. In navigation, you will frequently need to calculate the reciprocal of a given direction — that is, the direction that is 180° opposite to the direction given. So if you are travelling in a direction of 060°(T), the reciprocal direction is 240°(T). You simply add or subtract 180. If a runway has a direction of 353°(T), the reciprocal runway direction is 173°(T) — again, 353 plus or minus 180. That covers the sexagesimal system, true direction, the three-figure group rule, the runway exception, and reciprocal directions.