u03b1 which is zero for all positions the calculations are performed by using pure geometries and the deviations between the simulated measurements and the u201c real u201d measurements for each measurement view will generate a standard deviation value 3 u03c3 describing the in step 205 which indicate how well the stage correction function compensates for the actual appearance of the stage in step 206 the standard deviation value u2014 3 u03c3 is compared with a predetermined value and if 3 u03c3 is greater than the predetermined value i e 3 u03c3 is not ok the flow will proceed to step 208 where a new description of the stage s 1 x y u03b1 is calculated using deviations between the previously calculated simulated measurements m sim using the previous stage description s 0 x y u03b1 and the model of the plate p 1 x y and the actual measurements for each measurement view m new descriptions of the plate is thereafter calculated in step 209 using the new stage description s 1 x y u03b1 the average value for the measured positions in all measurement views is thereafter calculated for each position and a new model of the plate p 2 is calculated the flow is thereafter fed back to point 210 and steps 204 206 are repeated until the standard deviation 3 u03c3 is less than the predetermined value the flow ends in step 207 and the latest calculated description of the stage determines the correction function that will be applied to the stage fig3 illustrates an example of a standard deviation 3 u03c3 converging when performing the method.