The difference between the two corresponding angles formed by the intersection of a geodesic with two different meridians. Sometimes abbreviated to convergence or convergency. It may also be defined as the difference in forward and back azimuths at the two ends of a geodesic. At the equator, all meridians are parallel - they make the same angle, 90^o, with the equator. Passing north or south from the equator, two meridians converge until they meet at the poles, intersecting at angles equal to their differences of longitude. If the surface involved is spherical, the quantity is called spherical convergence; if it is a rotational ellipsoid (spheroid), it is called spheroidal convergence.