Half convergency correction

Half-convergency correction, also called the conversion angle, converts a great-circle bearing into the rhumb-line direction that can be plotted on a Mercator chart. A radio or direction-finding bearing travels a great circle and would plot as a curve on Mercator, so half the convergency between the two meridians is applied to straighten it: conversion angle equals half of d.long times sine of mean latitude. The correction is applied toward the equator, so the rhumb bearing lies on the equatorial side of the great-circle bearing. It grows with longitude difference and with latitude, vanishing on the equator and along a single meridian.