central meridian

The longitudinal line of symmetry of a map projection and, generally, a baseline for referencing the projection to the associated grid coordinate system. As such, grid north and true north are coincident along the central meridian, but at any other point of the projection, grid north and true north diverge as a function of geodetic latitude and longitudinal distance from the central meridian as a result of the convergence of the meridians. Most map projections portray meridian convergence, but one notable exception is the Mercator Projection.