Let us now understand about the Sexagesimal System of Direction Measurement which is actually used in aviation. We had understood about cardinal and quadrantal directions in our earlier lesson on measurement of directions in our earlier post.

The Cardinal and Quadrantal directions are widely used in Maritime Navigation. However for the purpose of Air Navigation, a more precise method of indicating directions is used in the form of Sexagesimal System. However, Cardinal points are retained as reference for Sexagesimal System.

Direction of the Geographic North Pole, which is defined as North, forms the Datum Reference for Sexagesimal System. The direction of North is referred as 000 or 360 direction. All other directions are represented as an angular distance like a circle from 0 to 360 degrees measured from North.

When you measure in the clockwise diection, East would be at 90 degrees angular displacement from North and hence referred as 090. Similarly, South has a numerical value of 180 and West has a value of 270. When we use the direction of Geographical North Pole or Actual North Pole as the datum of North it is called as True North. When True North is used as reference for Sexagesimal System, the system provides for True directions.

### RECIPROCAL DIRECTIONS

Reciprocal directions are in simple terms Opposite Directions. For example, the reciprocal of North is South and the reciprocal of East is West. When the same logic is applied to Sexagesimal System of reference, Reciprocal Directions are 180 degrees displaced from any given direction.

In order to obtain Reciprocal Directions, we need to add 180 to directions less than 180 and subtract 180 from directions greater than 180. For example, the reciprocal direction of 050 is 050 + 180 = 230 and reciprocal of 230 is 230 – 180 = 050.

In Air Navigation, you would be frequently required to calculate the reciprocal of directions. Hence, as student pilots you need to practice mental calculation of reciprocals accurately and quickly.

In the next lesson, we shall understand about the importance of Position Reference System. In order to understand about the position reference system, we need to start by understanding about Great Circles.