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i. The angle that one meridian on the earth's surface makes with another. It equals the angular difference between the measurements of the great circle at each meridian.
ii. The angle between two meridians at a given latitude. At the poles, its value is the change of longitude, but it slowly reduces toward the equator when the value is zero, as meridians are parallel to one another. Earth's convergence = Change of longitude (ch long) × Sine latitude.
iii. The difference between the great-circle bearings of two meridians. The angle (b − a) in the figure in (i) is the earth's convergence = ch long × sine lat.
iv. Also called earth's convergence.