Colors are used to represent curvature values, and from these various color-coded curvatures, corneal shapes are categorized. The color-coded display generated by the computer is a dioptric map, with each color representing a specific dioptric value relative to the corneal radius of curvature. With some systems, you can adjust the dioptric gradations from one color to the next. The regular corneal surface is really an asymmetric asphere, like an off-center ellipse with toricity incorporated. The normal cornea typically results in a map with an aspheric and asymmetric topography.
Some instruments refer to or model the cornea as a spline curve.
Classically, the cornea is steepest centrally and flattens toward the periphery. Maps of both eyes of a normal individual, although similar, will not necessarily be mirror images of one another. There can be variations in normal corneal topography, but they are typically round or oval. The two halves of the bow tie are usually equal in size and 180 degrees apart. Cataract surgery, penetrating keratoplasty and trabeculectomy can also result in asymmetric oblique bow tie patterns or irregular corneal maps.