Techniques are disclosed in which a topographic parameter is determined in each semi-meridian of the eye by considering the topography in each of three concentric zones from the central axis at 3 mm, 5 mm, and 7 mm and assigning weighting factors for each zone, By selectively treating the weighted values in the three zones, parameters of magnitude and meridian can be obtained for each semi-meridian. From these parameters, a single topographic value for the entire eye (CorT) can be found as well as a value representing topographic disparity (TD) between the two semi-meridians. The topography values for the semi-meridians are used in a vector planning system to obtain treatment parameters in a single step operation.
