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On a polar stereographic chart, a straight line is drawn from A to B. The position of B is 80°S 010°W. Initial track at A is 103°T, and the final track at B is 060°T. What is the longitude of A?
  • A
    012°E
  • B
    032°W
  • C
    053°W
  • D
    033°E

Refer to figure.

Convergency (or the angle of convergence) represents the difference between the initial and the final Great Circle track.
On a Polar Stereographic: Convergency = Change Longitude
The convergency between two positions, or the meridians running through these positions, is therefore governed by the difference in longitude.
The learning objectives state that you should know the formula for convergency, which is as follows:

  • Convergency = Change in Longitude
    OR
    Convergency = GC Initial True Track - GC Final True Track


Initial track at "A" = 103º and Final track at "B" = 060º => Track direction decreases;

  • In the Southern Hemisphere, we can say that:
    Flying westerly directions, the track direction increases; and
    Flying easterly directions, the track direction decreases.

Which means that, in this case, we are flying an eastbound track. Consequently, "B" will be to the east of "A".
Change of longitude = Convergency
And Convergency = 103º - 060º = 43º

=> Therefore, position "A" is at meridian 43º + 010ºW = 053ºW.

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