Convergency:
Convergency refers to the angle formed at the pole by the intersection of two meridians on the Earth’s surface as they converge from the equator to the pole.

• At the Equator, meridians are parallel, so convergency is zero.
• At the Pole, meridians make the maximum angle, which equals the change in longitude.
• Convergency varies with latitude and is proportional to Sin(latitude).

Conversion Angle (CA):
The Conversion Angle is the angular difference between the Rhumb Line (RL) and Great Circle (GC) bearing or track.

• CA = Convergency / 2
• The relationship between GC and RL depends on the hemisphere and the bearing or direction of travel.