When compared to meridional maps, axial features are smoother and larger in extent, which is a direct consequence of the radial smoothing via averaging. Historically, axial curvature and its traditional color contour maps have been known by several other names. It was called global, to differentiate it from meridional curvature, which is local. It was called a dioptric or power map, because of the ophthalmic tradition of expressing curvature in keratometric diopters via the paraxial power relation. Corneal Topography .
Meridional curvature is local surface curvature measured in meridional planes .
Historically, meridional curvature and its traditional color contour maps have been known by several other names. It was called tangential, because tangential optical planes are often meridional planes. It was called instantaneous or local to differentiate it from the radially-averaged axial curvature . Corneal Topography . Radial curvature is a local surface curvature measured in radial planes , which are surface normal planes lying perpendicular to traverse planes .