Corneal Topography surface height measured relative to some symmetric and close-fitting reference surface . Elevation is positive when the data surface is anterior to the reference surface. Corneal Topography elevation in the view-axis direction measured from the data surface to some reference surface.
Axial elevation is used with non-spherical references surfaces.
Corneal Topography elevation measured from the data surface to some reference surface along a local data-surface normal . Corneal Topography elevation measured from the data surface to some reference surface along the intercepting reference-surface normal. Reference elevation is used with spherical reference surfaces. This common case is also known as radial elevation, because elevation differences are directed centrifugally from the sphere center.