Polar charts, transverse Mercator and grid
Navigation · Chapter 13 · DGCA CPL and ATPL
The charts used near the poles and for long north–south areas, and how grid navigation replaces true north there.
Remember
- Polar stereographic: straight meridians, circle parallels, n = 1, scale best at the pole.
- Above about 70° a straight line is a good great circle.
- Straight-line track on one parallel: (180 − ch long) ÷ 2.
- Transverse Mercator: touches a meridian; parallels are ovals; for long, narrow north–south areas.
- Grid: convergence east true least, west true best.
Why do the polar regions need their own chart?
Why a polar chart
- Mercator and Lambert break • Mercator cannot show the poles at all; Lambert is weak there • Near a pole the meridians crowd together: "north" changes every few miles
- A plane on the pole • Lay a flat sheet on the North Pole • With the light at the centre, the grid spreads out fast away from the pole
- Move the light to the South Pole • Shine from the opposite pole instead • The grid now spreads more gently: this is the polar stereographic chart
- Seen from above • Meridians: straight lines from the pole • Parallels: circles round it
- Where the scale holds • Scale is correct at the pole and grows outwards • Within 1 % out to 78°, within 3 % out to 70° • So above about 70° a straight line is a good great circle
What are the numbers to know about a polar stereographic chart?
The polar chart in use
- A plane on the pole • Meridians: straight lines from the pole • Parallels: circles round it • This is the polar stereographic chart
- Scale is best at the pole • Within 1 % of correct out to 78°
- Convergence is the whole ch long • n = 1: chart convergence = ch long • Correct only at the pole
How do you work out a straight-line track on a polar chart?
Lines and tracks near the pole
This lesson is part of the Navigation plan. See plans
What happens if you turn the Mercator cylinder on its side?
Transverse and oblique Mercator
This lesson is part of the Navigation plan. See plans
How do you navigate when the meridians all crowd together?
Grid navigation
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 13
Test yourself
This lesson is part of the Navigation plan. See plans