Grid Navigation Technique

Wg Cdr Rajagopal

Grid Navigation

Index

Introduction to Grid Navigation

  • Grid navigation helps in avoiding the steering problems at high latitudes
  • True or magnetic directions are unusable in in high latitudes due to 3 reasons
  • Rapid convergence causes true track to change rapidly
  • Magnetic variation is high and varies considerably
  • Magnetic compass is ineffective due to lesser horizontal component

Grid Charts

  • Grid charts are created by nominating a datum meridian and calculating directions throughout the chart with the datum meridian
  • Grid charts have a square grid instead of latitude longitude graticules
  • Grid charts are used at high latitudes near polar regions
  • Greenwich meridian is normally taken as a datum meridian and other grid meridians are drawn parallel to the datum meridian

Grid Convergence

  • On a Grid chart, grid convergence can be calculated as the angular difference between True and Grid direction
  • Longitudinal difference between the Meridian of a place and the datum meridian would help in calculating grid convergence

Grid Convergence in the Southern Hemisphere

  • In the southern hemisphere, convergence is same as orientation of meridian
  • Convergence is west if meridian of place is west of datum meridian
  • Convergence is east if meridian of place is east of datum meridian

Grid Convergence in the Northern Hemisphere

  • In the northern hemisphere, convergence is opposite to orientation of meridian
  • Convergence is east, if meridian of place is west of datum meridian
  • Convergence is west, if meridian of place is east of datum meridian

Standard Polar Grid Chart

  • Standard polar grid chart has Greenwich meridian as datum meridian
  • Isogrivs are lines joining places of equal grivation
  • Grid convergence is the angular difference between true and grid north
  • Grid charts use the principle of grid convergence

Chart Convergence

  • Chart convergence in grid charts follows the same rules as in other charts
  • In grid charts using Lambert’s projection
  • Chart convergence = Change Of Longitude X Constant Of Cone
  • In grid charts using Polar stereographic projection
  • Chart convergence = Change in Longitude

Convergence

  • Convergence depends on the position of True North and grid north
  • Convergence is east when True North lies to east of grid north
  • Convergence is west when True North lies to west of grid north
  • The following rules apply to convert Grid direction to true direction
  • If Convergence is east, true track is least
  • If Convergence is west, true track is best

Grid Steering Using Magnetic Compass

  • Grivation is used to directly convert grid track to magnetic track
  • Grivation is the algebraic sum of convergence and variation
  • Grid steering can be done using magnetic compass by applying grivation

Calculation of Magnetic Tracks

  • Grid track does not change direction in a grid chart
  • Other tracks follow the rule of heading conversion
  • Grid – Convergence – True – Variation – Magnetic – Deviation – Compass
  • Grivation = Convergence + Variation

Best of luck