Measurement of Directions

Wg Cdr Rajagopal

Measurement of Directions in Aviation

Index

Shape of the Earth

  • The shape of earth is called as an Oblate Spheroid
  • The shape is also called as a Geoid which means Earth-Shaped
  • Oblate Spheroid can be considered as an imperfect sphere
  • Spherical with slight flattening at the ends of rotation
  • The end points of rotation of spin axis are called poles
  • Flattening of the earth at the poles is called compression

Compression Ratio

  • Compression of Earth is given by a ratio called Compression Ratio
  • The difference between Equatorial and Polar diameter is 43 Km or 23 NM
  • Formula for Compression Ratio is
  • (Equatorial Diameter – Polar diameter) / Equatorial diameter
  • Value of earth compression is 1/300 or 0.3%

World Geodetic System (WGS 84)

  • ICAO accepted World Geodetic System (WGS 84) as Standard model for Civil Aviation
  • To create maps and charts we need to create a mathematical model of earth
  • In the past, different mathematical models of earth were used by various countries for their map preparation
  • These models were optimised for their particular region
  • However they are unsuitable for world wide navigation systems
  • International Civil Aviation Organisation (ICAO) standardized the model to WGS 84
  • WGS 84 considers earth to be a perfect sphere for the purpose of Navigation
  • Therefore, Spherical Trigonometry can be used for navigational calculations

Spin Axis of Earth

  • Spin or Polar axis is the axis of rotation of the earth
  • Earth rotates around its axis as well as revolves around the sun
  • Poles are the extremities of the axis about which earth spins
  • Orbital plane is the plane of revolution of earth around the sun
  • Spin axis of the earth is tilted at an angle of 23.5 deg to the Orbital plane

Cardinal Directions of East and West

  • Directions of North, South, East and West are called the Cardinal directions
  • East is defined as the direction into which earth rotates
  • West is defined as the direction from which earth rotates

Cardinal Directions of North and South

  • Geographical North and South poles are the two ends of spin axis of earth
  • The pole from over which earth appears to rotate anti-clockwise is called as the North Pole
  • Direction of travel to reach geographical North pole is True north
  • The pole from over which earth appears to rotate clockwise is called as the South Pole
  • Direction of travel to reach geographical South pole is True south

Quadrantal Directions

  • North east, South east, North west and South west are called as Quadrantal directions
  • Quadrantal directions divide the earth into four quadrants

Sexagesimal System

  • Directions used in aviation are measured using Sexagesimal system
  • Sexagesimal means in multiples of 60
  • In the Sexagesimal system directions are measured in degrees Clockwise from North
  • True Directions are measured by taking True north as datum of measurement

Directions in Sexagesimal System

  • In the Sexagesimal system of measurement directions are given in 3 digits
  • For example, North is referred to as ‘zero zero zero’ or ‘Three Six Zero’
  • Likewise, 090 is East, 180 is South and 270 is West

Reciprocal Directions

  • Opposite directions are called Reciprocal directions
  • Reciprocal of North is South and that of East is West
  • To calculate reciprocal directions in the sexagesimal system
  • Add 180 to values less than 180
  • Subtract 180 for values greater than 180

Best of luck