Convergency and Conversion Angle

Convergency Explained

Convergency in navigation refers to the angle by which meridians (lines of longitude) converge as you move north or south from the equator. This is crucial when plotting courses over long distances, especially at higher latitudes, where the convergence of meridians is more pronounced. The standard convergency formula is:

Convergency = Change of Longitude × Sine of Mean Latitude

For Lambert Conformal charts, the formula adapts to:

Convergency = Change of Longitude × Sine of the Parallel of Origin

The 'parallel of origin' is typically the latitude halfway between the chart's standard parallels.

Conversion Angle Defined

The conversion angle is the angular difference between a great circle track and a rhumb line track at any point along the route. It is calculated as:

Conversion Angle = ½ × Convergency

This angle is vital for converting between great circle and rhumb line tracks, allowing pilots to understand how their heading needs to change when switching between these navigation methods.

Practical Application

• On a great circle, the initial and final tracks differ due to convergency; this is especially significant over long distances or at high latitudes.
• On a Lambert Conformal chart, the convergency formula uses the sine of the parallel of origin, reflecting the chart's projection properties.
• On a Polar Stereographic chart, convergency equals the change in longitude, as the convergence factor (sine of 90°) is 1.

Key Differences

• Convergency is the total angular change between meridians (track convergence) between two points.
• Conversion angle is the local difference between great circle and rhumb line tracks at a point, always half the convergency.

Understanding these concepts ensures accurate plotting, course corrections, and safe navigation across various chart types.