When navigating with aeronautical charts, understanding chart convergency is important. This refers to the angle between the meridians (longitude lines) on a chart and the true north-south lines of the Earth.

The convergence indicates how much the track changes per degree of longitude traveled and varies with latitude.


Earth Convergency vs. Chart Convergency


Earth Convergency: Describes how the true bearing along a great circle track changes.

Chart Convergency: Describes how the track bearing of a straight line on a chart changes as you move.


Convergency Across Different Chart Projections


Lambert Conformal Conic Projection:

  • Projection it is change of longitude multiplied by the sine of the mean latitude - it equals Earth convergency at the (PO) Parallel of Origin but otherwise remains constant everywhere.
  • The sine of the PO is called the constant of the cone - as the meridians are straight, the value is constant over the chart, instead of changing between latitudes, as it would on the Earth.

Polar Stereographic Projection:

  • Chart convergency equals the change in longitude, as meridians are straight lines converging at the poles.
  • This projection reflects longitude changes without distortion.

Mercator Projection:

  • There is no chart convergency because the meridians are parallel lines.
  • Useful near the Equator but less accurate at higher latitudes.

Check out our video about the topic!