Rule 39C

CDMVT in Air Navigation


True Direction: The direction towards North pole of a meridian is called True North.

–    The direction is measured through 360º clockwise from the True North.

–    It means True North being 000º East is 090º, South 180º and West 270º.

–    Cardinal Directions:      North-East-South-West.

–    Quadrantal Directions:  North-East (045) South-East (135), South-West (225) and North-West (315).

–    True North is Indicated by a diamond at the head of direction vector.

Magnetic Direction: The direction of magnetic meridian at a place towards North magnetic pole is called Magnetic North.

–    Angles measured with respect to magnetic North through 360º clockwise is called magnetic direction.

–    The difference between True North and Magnetic North is due to earth’s North pole and magnetic North pole not being coincident.

–    Magnetic North is indicated by semi-diamond at the head of the direction vector.

Variation: Angular difference between True North and Magnetic North (true direction and magnetic direction) is called Variation and it is said to be East or West as the Magnetic North lies to the East or West of the True North.

  • Variation changes over a period. The rate of change of Variation is indicated along the Variation lines or on the legend of the projection.
  • Lines joining places of equal Variation are called Isogonals
  • Line joining places of zero Variation is called Agonal
  • Variation varies from 0º to 180º.

Compass Direction: The direction indicated by North seeking pole of a freely suspended magnet is called Compass North.  The direction measured through 360º clockwise from Compass North is called compass direction.

–    Aircraft magnetism is the main cause of difference between magnetic direction and compass direction.

–    Compass North is indicated by a circle at the head of the direction vector.

Deviation: The angular difference between Magnetic North and Compass North (magnetic direction and compass direction) is called Deviation and is said to be East or West as the Compass North lies to the East or West of the magnetic North.

















Conversion of Direction:  A simple method, called CDMVT method may be used.

Compass                Deviation                          Magnetic                          Variation                          True

                                                            →                                                            →

C                                  D                                     M                                        V                                       T


+E                                                        +E

-W                                                        -W


050                           7E                                   057                                        4W                                053


002                         3W                                   359                                        2E                                  001


C to M to T:  E is +ve and W is -ve.

T to M to C: E is -ve and W is +ve













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